Victor Figueira Arcuri
DESIGN AND MODELLING OF A THERMAL STORAGE SYSTEM FOR AIR CONDITIONING APPLICATIONS
Dissertação submetida ao Programa de Pós-Graduação em Engenharia Mecânica da Universidade Federal de Santa Catarina para a obtenção do Grau de Mestre em Engenharia Mecânica. Orientador: Prof. Jonny Carlos da Silva, Dr. Eng.
Co-Orientador: Prof. Saulo Ghuts, Dr. Eng.
Florianópolis 2019
Victor Figueira Arcuri
DESIGN AND MODELLING OF A THERMAL STORAGE SYSTEM FOR AIR CONDITIONING APPLICATIONS
Esta Dissertação foi julgada adequada para obtenção do Título de “Mestre em Engenharia Mecânica”, e aprovada em sua forma final pelo Programa de Pós-Graduação em Engenharia Mecânica.
Florianópolis, 28 de Fevereiro de 2019.
___________________________ Prof. Jonny Carlos da Silva, Dr. Eng -
Coordenador do Curso
___________________________ Prof. Jonny Carlos da Silva,
Dr. Eng - Orientador ___________________________ Prof. Saulo Güths, Dr. Eng. – Coorientador Banca Examinadora: ___________________________ Prof. Cesar José Deschamps, Dr. Eng.
___________________________ Prof. Rodrigo de Souza Vieira, Dr. Eng.
ACKNOWLEDGEMENTS
To CNPq and UFSC mechanical engineering postgrad program, my acknowledgment for financial support. Also, to FAPESC and Sinapse da Inovação program, for financial support together with DIEL Energia LTDA.
A salutation to members of NEDIP, as coworkers, providing unconditional mental and technical support. To my advisor Prof. Dr. Eng. Jonny Carlos da Silva, my sincere respect and thank for his knowledge and professionalism.
To members of Labtermo, in special my co-advisor Prof. Dr. Eng Saulo Guths and Eng. Edevaldo Brandílio Reinaldo, my sincere thanks to infrastructure and technical knowledge shared. To members of LabTucal for their infrastructure and technical support.
At last, to members of DIEL Energia LTDA, in special, my brother Bruno Arcuri, for pushing me to my best all the time. To my family and friends for love and psychological support.
RESUMO
Em um cenário de grande aderência às tecnologias renováveis e aumento da demanda por refrigeração devido a mudanças climáticas e aquecimento global, tecnologias de termoacumulação (TES) representam um campo crucial. Assim, a aplicação de uma metodologia de projeto para criar novas tecnologias relacionadas à equipamentos de ar condicionado e armazenamento de energia pode ser visto como uma grande oportunidade de inovar e trazer soluções práticas ao mercado. Este trabalho é focado especificamente nas etapas de projeto conceitual e projeto preliminar do processo de desenvolvimento de produto (PDP), com a intenção de desenvolver e validar o conceito de um sistema de armazenamento de energia térmica capaz de suprir 12.000BTU/hr de capacidade frigorífica para um ar condicionado durante 3 horas. Portanto, um número de soluções técnicamente viáveis foi criado, avaliado, e posteriormente identificado a melhor opção dentre eles. Além disso, a metodologia de validação de conceito foi baseada em ambos modelo físico experimental e computacional. Primeiramente, foram feitos dois protótipos como prova de conceito, buscando comprovar o fenômeno físico proposto. Em sequência, um protótipo funcional em escala foi construído, com um planejado sistema de aquisição de dados. Por último, um modelo computacional foi desenvolvido em ambiente Simcenter Amesim®. Como resultado, foi possível comparar os modelos fisico e computacional, apontando melhorias para serem implementadas na etapa de projeto detalhado. Assim, a simulação parametrizada permite avaliar operações do sistema variando parâmetros e arranjos do equipamento. Por fim, a pesquisa comprova a efetividade da utilização do PDP para o desenvolvimento de designs inovadores, além de explorar os campos de modelagem e simulação.
Palavras-chave: Armazenamento de energia, ar condicionado, modelagem, processo de desenvolvimento de produto, refrigeração.
RESUMO EXPANDIDO
Introdução
Temperaturas altas unidas ao uso intensive de ar condicionado são os maiores responsáveis pelo aumento de demanda elétrica no Brasil durante o verão, o que causou cortes de energia devido a falta de capacidade para suprir a demanda (EPE, 2015).
Além disso, de acordo com o MME (2017), a economia ganha pela implementação do horário de verão é diluída devido ao alto gasto de energia no ar condicionado. O relatório afirma que o governo brasileiro pretende exigir sistemas de ar condicionado mais eficientes dos fabricantes. No entanto, especialistas acreditam que essa ação não pode mudar o cenário por si só, especialmente se comparado a tecnologias adotadas em outros países. Além disso, ABRAVA (2014) afirma que no Brasil em 2013, mais de 4 milhões de unidades de ar-condicionado foram comercializadas, com 74% delas do tipo split.
Portanto, em um cenário de mudanças climáticas e aquecimento global, a aplicação da metodologia de design para criar novas tecnologias relacionadas ao ITES (Armazenamento térmico em gelo) pode ser vista como uma grande oportunidade para inovar e trazer soluções viáveis para o mercado.
Um planejamento de produto anterior foi realizado, idealizando uma bateria de gelo suficiente para suprir 12.000 Btu / hora de refrigeração durante 3 horas, com um calor latente total de 36.000 BTUs armazenados. O objetivo é produzir gelo fora do horário de pico, armazená-lo em um banco de gelo e usá-lo para esfriar durante o horário de pico, garantindo o desempenho necessário para gerar economia para um possível cliente. Este trabalho avalia uma possível solução da tecnologia ITES para maior integração de escala no setor comercial brasileiro, entendendo os aspectos tecnológicos da solução, desenvolvendo um protótipo e modelando ainda mais o sistema no LMS Imagine.Lab Amesim® 1D para captar a nova tecnologia criada, por realizando uma comparação entre o modelo físico e computacional.
Objetivos
Esta pesquisa tem como objetivo avaliar possíveis soluções da tecnologia ITES para maior integração de escala no setor comercial do Brasil, para entender os aspectos tecnológicos da solução, desenvolvendo um protótipo e modelando ainda mais o sistema no LMS Imagine.Lab
Amesim® 1D para absorver a nova tecnologia criada . Assim, realizando uma comparação entre o modelo físico e computacional e uma outra parametrização de modelos.
Metodologia
Segundo Dias et al. (2011), um método para obter uma estrutura de função é uma função global. A partir de uma função global, que é a principal tarefa que o sistema executará, é possível entender melhor os fluxos de energia, sinal e material envolvidos no processo.
Entendendo o funcionamento básico do sistema em design, a função global pode ser dividida em subfunções, com menos complexidade. Utilizando esta técnica para este problema, a partir da função principal “armazenar energia térmica”, identificou-se a necessidade de três subfunções, que foram posteriormente quebradas em funções elementares. O resultado é uma árvore de funções com três níveis de funções.
A partir das funções de nível inferior, é possível pesquisar e aplicar princípios de trabalho para o projeto da bateria de gelo. Além disso, Back et al (2008) afirma que os princípios de trabalho podem ser gerados por diferentes métodos, como brainstorming e benchmarking. Portanto, os produtos existentes disponíveis no mercado foram analisados e sintetizados, a fim de ampliar o campo do conhecimento.
Portanto, uma metodologia foi previamente estabelecida utilizando matriz morfológica, a fim de criar uma série de soluções diferentes. Na sequência, realizando duas rodadas de avaliação da matriz de Pugh, foi possível identificar o conceito mais adequado para a função global “armazenar energia térmica” para aplicação de CA. Essa etapa marca o fim da fase de projeto conceitual e o início do projeto preliminar. Assim, o processo de design é continuado com uma melhor investigação do conceito mais adequado, validando e otimizando-o.
O início do projeto preliminar é o conceito de um produto técnico. A partir deste estágio, o projeto é desenvolvido de acordo com critérios técnicos e econômicos, buscando detalhes que possam levar diretamente à produção. A metodologia de modelagem é então explorada, a fim de criar evidências, o que valida o projeto proposto. Além disso, a validação de conceito é dividida em duas etapas: Prototipagem e modelagem computacional.
O protótipo desenvolvido deve fornecer uma compreensão completa dos requisitos do produto necessários, dentro de um custo adequado. Nesta pesquisa, um protótipo físico foi construído com uma
aquisição de dados planejada para melhorar a eficácia do teste. Uma bancada de teste foi projetada para acomodar todos os elementos e fornecer um melhor arranjo.
Uma vez tendo resultados experimentais, é possível parametrizar a simulação, criando um modelo muito conciso. Assim, a simulação se torna um caminho interessante e mais barato para dimensionamento e otimização. Além disso, a simulação visa modelar o sistema com os parâmetros do protótipo construído para avaliar os diferentes parâmetros e estratégias de operação do sistema. A Figura 3 ilustra o sistema modelado em Amesim, onde pode ser visto um sistema de carga, que consiste em um sistema de compressão de vapor que remove o calor da água. Por outro lado, há um sistema de descarga, composto por uma bomba de refrigerante, um evaporador de CA e uma bobina de descarga trocando calor com o gelo formado. No artigo final, esse modelo será descrito em detalhes.
Resultados e Discussão
Entre os resultados do protótipo e simulação, a comparação entre a diminuição da temperatura no modo de carga da bateria e o tempo de descarga da bateria podem ser destacados. As curvas mostradas sugerem uma diferença na capacidade de refrigeração entre os ciclos, onde o ciclo de carregamento de protótipos alcança de 20 ° C a 0 ° C uma hora antes do ciclo de carregamento da simulação. Esta divergência na capacidade de resfriamento não é causada apenas por limitações nos submodelos de troca de calor dos tanques, mas principalmente devido à complexidade da simulação refletindo no número de parâmetros para ajuste. Em outro caso apresentado, a curva de descarregamento do protótipo apresenta um pequeno aumento da temperatura da água após 3 horas de operação. A curva da simulação, por outro lado, apresenta um aumento posterior na temperatura, mas com uma inclinação maior. Esse contraste pode ser explicado por um derretimento gradual de gelo dentro do tanque, em vez de uma mudança repentina de estado.
Considerações Finais
Esta pesquisa teve como objetivo avaliar possíveis soluções da tecnologia ITES para condicionadores de ar split, que podem ser integrados no setor comercial brasileiro. O estudo aplicou técnicas de desenvolvimento de produtos através de fases conceituais e preliminares de projeto. Em relação ao protótipo construído e modelo de simulação, a
divergência é explicada principalmente devido a um grande número de parâmetros e, portanto, uma maior complexidade para garantir a eficácia de todos os parâmetros adotados. Além disso, a pesquisa foi resultado da dissertação de mestrado do programa de pós-graduação em engenharia mecânica da UFSC, financiada pelo patrocínio da CAPES e pelo programa de subsídios da FAPESC “Sinapse da Inovação IV” para startups.
Palavras-chave: Armazenamento de energia, ar condicionado, modelagem, processo de desenvolvimento de produto, refrigeração
ABSTRACT
In a scenario of high penetration of renewables and increasing demand for cooling due to climate change and global warming, TES (Thermal Energy Storage) plays a crucial role. Therefore, the application of design methodology to create new technologies related to air conditioning applications and energy storage can be seen as a great opportunity to innovate and bring feasible solutions to market. This work is focused specifically in the conceptual and preliminary design phases of the product development process (PDP), in order to develop and validate a concept of a thermal storage system capable to supply a 12,000 BTU/hr air conditioning for 3 hours. Therefore, a number of feasible concepts were created, evaluated, and later identified a best concept. Furthermore, the concept validation methodology was based in both physical and computational models. Firstly, two proof of concepts prototypes were developed, in order to prove the physical phenomena involved. In addition, a functional scale prototype was built with a planned data acquisition. Lastly, a computational model was developed in Simcenter Amesim® environment. As a result, it was possible to compare physical and computational models, suggesting improvements to be carried to detailed design. Also, a parameterized simulation allows to evaluate different system operation parameters and strategies. In conclusion, the research proves the effectivity of PDP for an innovative design, besides explore modelling and simulation field.
Key words: Energy storage, air conditioning, modelling, product development process, refrigeration.
LIST OF FIGURES
Figure 2.1 - PRODIP Methodology ...16
Figure 2.2 – Function Tree ...17
Figura 2.3 – Example of a combination of principles – Morphological matrix .18 Figure 2.4 – Example of a Pugh matrix ...19
Figure 2.5 – System using a compressor...22
Figure 2.6 – Ductless mini split-system ...24
Figure 2.7 – Refrigeration cycle of a vapor compression system ...25
Figure 2.8 – R-22 AC split-system at ASHRAE conditions ...26
Figura 2.9 – Hybrid cooling solution, combining a UTSS and a packaged DX system ...30
Figure 3.1 – Global Function ...32
Figure 3.2 - Function tree of an ice thermal storage system ...32
Figure 3.3 – Function-means Tree of available ITES solutions ...35
Figure 3.4 – Magnetic refrigeration process graph ...39
Figure 3.5 – Absorption refrigeration process graph ...40
Figure 3.6 – Thermoelectric refrigeration process graph ...41
Figure 3.7 – Morphological Matrix with examples of combinations ...43
Figure 4.1 – Discharge mode scheme ...51
Figure 4.2 - – Discharge mode experiment ...51
Figure 4.3 – Charge module scheme ...52
Figure 4.4 – Charge module experiment ...53
Figure 4.5 – Ice boundary measurement ...54
Figure 4.6 – Ice boundary measurement ...55
Figure 4.7 – Alpha prototype ...56
Figure 4.8 – Coil-in-coil design ...57
Figure 4.9 – Sequence of condensation process at pump suction in different state flows. (a) Stationary Flow, (b) Slug Flow, (c) Bubbly flow, (d) Single-phase liquid flow ...58
Figure 4.10 – Superior view of ice formation in system coil, (a) First distribution configuration, (b) Second distribution configuration. ... (a) (b) ...59
Figure 4.11 – Defect in spiral and distribution tube joint ...60
Figure 4.12 – Boundary conditions of flow simulation ...60
Figure 4.13 – Flow trajectories inside the tubes ...61
Figure 4.14 – Charge mode validation at Beta prototype ...62
Figure 4.15 – Full Model representation in Amesim ...64
Figure 4.16– Water sub-model causality ...67
Figure 4.17 – Water signal equation ...67
Figure 4.18– Power flow in compressor component ...68
Figure 4.19 – High side pressure of charge system model ...69
Figure 4.20 – Non-adiabatic capillary tube and suction line model ...70
Figure 4.21 – Power flow in capillary tube component ...71
Figure 4.23 – Pump and evaporator model at discharge system ... 72
Figure 4.24 – Discharge coil model ... 73
Figure 5.1 – Compressor discharge and condensing temperatures in prototype and simulation tests – One-hour operation ... 75
Figure 5.2 - Compressor suction and evaporating temperatures in prototype and simulation tests – One-hour operation ... 75
Figure 5.3 – Pressure-enthalpy diagram for R134a cycles ... 77
Figure 5.4 - System first charge comparison ... 78
Figure 5.5 – Pump suction temperature in both tests ... 79
Figure 5.6 – Pump Discharge temperatures in both tests ... 79
Figure 5.7 – Returning gas temperature in both tests ... 80
Figure 5.8 – Water temperature - Time of discharge ... 81
Figure 5.9 – Mass flow rate curve in discharge mode simulation... 81
Figure 5.10 - Pressure output at compressor discharge ... 82
Figure 5.11 – Cooling capacity with pressure control ... 83
Figure 5.12 – Cooling capacity with pressure control with zoom in curves behavior ... 83
Figure 5.13 - Comparison with and without heat exchange in expansion. ... 84
LIST OF TABLES
Table 2.1 – The Development phases of a Hardware product ...20
Table 2.2– PCMs feasible to AC application ...28
Table 2.3 – Classification and conditions of ITES technologies ...29
Table 3.1 - Functions of ice thermal storage system ...38
Table 3.2 – Working principle combination ...44
Table 3.3 – Pugh Matrix, First round ...47
Table 3.4 – Pugh Matrix, Second round ...47
Table 4.1 – Water properties ...66
LIST OF ABBREVIATIONS AND ACRONYMS
PRODIP Integrated Product Design Methodology
(projeto integrado de produtos)
CTES Cool Thermal Energy Storage
ITES Ice Thermal Energy Storage
CWS Chilled Water Storage
ToU Time-of-use Tariff
PCM Phase Change Material
AC Air-Conditioning
HVAC Heating, Ventilation and Air-Conditioning
DX Direct Expansion
UTSS Unitary Thermal System Storage
COP Coefficient of Performance
TABLE OF CONTENTS
1 INTRODUCTION ... 11
1.1 Background ... 11
1.2 Aims and objectives ... Erro! Indicador não definido. 1.3 Work Structure ... 14
2 LITERATURE REVIEW ... 16
2.1 Product Development Process ... 16
2.1.1 Conceptual Design ... 17
2.1.2 Preliminary Design ... 20
2.2 Modelling and simulation ... 21
2.2.1 Dynamic simulation ... 21
2.3 Air Conditioning ... 23
2.4 Cold Thermal Energy Storage (CTES) ... 27
2.4.1 Sensible and latent heat systems ... 27
2.4.2 ITES Technologies ... 28
3 CONCEPT DESIGN METHODOLOGY ... 31
3.1 Establishing the Function Structure ... 31
3.1.1 Global Function ... 31
3.1.2 Function Tree ... 32
3.1.3 Existing products / Benchmarking ... 33
3.2 Concept development ... 37
3.2.1 Searching for Working Principles ... 37
3.2.2 Creating solutions ... 42
3.2.3 Selecting the winner concept ... 44
4 CONCEPT VALIDATION METHODOLOGY ... 50
4.1 Prototyping ... 50
4.1.1 Proof of Concepts ... 50
4.1.2 Development of an Alpha prototype ... 54
4.2 Computational model ... 62
4.2.1 PCM modelling ... 66
4.2.2 Charge system modelling ... 68
4.2.3 Discharge system modelling ... 72
5 RESULTS AND DISCUSSION ... 74
5.1 Charge Mode Comparison ... 74
5.2 Discharge Mode Comparison ... 78
5.3 Performance study ... 82
6 CONCLUSION ... 86
6.1 Work objectives ... 86
6.2 Future works ... 87
APPENDIX A – PRESSUTE ENTALPHY DIAGRAMS ...100 APPENDIX B – WORKING PRINCIPLES COMBINATION ...102 APPENDIX C – SUBSMODELS AND PARAMETERS ...110
1 INTRODUCTION
High temperatures and the intensive use of air conditioning are considered the main drivers of the increase in Brazil’s electricity demand in the summer, which has been causing major power cuts due to insufficient supply to meet demand (EPE, 2014; EPE, 2015a). Moreover, according to Darwall (2015), utilities and grid operators often argue that
variable wind and solar supplies offer risks of failure to today power systems, due their exposure to weather risk. Thus, if significant proportion of the generating capacity comes from intermittent renewables, the storage of the excess energy generated can avoid major power cuts.
Therefore, in a scenario of climate change and global warming, the
application of design methodology to create new technologies related to
CTES (Cool Thermal Energy Storage) can be seen as a great opportunity
to innovate and bring feasible solutions to market.
1.1 Background
According to the IPCC (2014), global air conditioning energy demand will grow from 300 TWh in 2000 to more than 10,000 TWh in 2100. Most of the growth will come from developing countries, where demand is driven by temperature (predominantly hot climates) and income (Peters & Strahan, 2016). By 2060, energy demand for space cooling is expected to overtake the demand for space heating, as cooling demand in developing countries of the global south grows faster than heating demand in the developed northern economies (Isaac & van Vuuren, 2009). A study by Davis & Gertler (2015) points the 12 countries in the world with the most “air conditioning potential”. Excluding the U.S., the list is dominated by middle and low-income countries with warm climates correlating the growth in energy consumption with weather and income growth. While the number of U.S. homes equipped with air conditioning rose from 64 to 100 million between 1993 and 2009, 50 million air-conditioning units were sold in China, in 2010 alone (Cox, 2012). In India, the number of room air conditioners rose from 2 million
in 2006 to 5 million by 2011 and is expected to reach 200 million by 2030 (IGSD, 2013). In the EU, cooling demand in buildings is expected to rise 70% by 2030 (Birmingham Energy Institute, 2016) and countries are already struggling to keep up with peak power demand in hot weather.
In Brazil, the numbers are also rising. ABRAVA (2014) states that in 2013 more than 4 million air conditioning units were marketed, being 74% of them split type. Furthermore, according to the journal A Tribuna (2017), the Ministry of Mines and Energy (MME) points that the economy gained by the implementation of the summer time is diluted due to high spent of energy in air conditioning. The reporting affirms that Brazilian government intends to require more efficient air conditioning systems from the manufactures. However, specialists believe that this action cannot change the scenario by itself, especially if compared with technologies adopted in other countries.
The growth of air conditioning use connects directly with energy demand. To reduce peak electricity demand in cases of intensive air conditioning use, Spataru et al. (2014) suggests that Cool Thermal Energy Storage (CTES) can be used to shift building cooling load to off-peak time. There are mainly three types of CTES: Chilled Water Storage (CWS), Ice Thermal Energy Storage (ITES) and Eutectic Salt Thermal Energy Storage, being the first two the most promising alternatives for general use (Hasnain, 1998; Yau & Rismanchi, 2012; ASHRAE, 2007). In urban environments, where space is a concern, ITES is the best choice to serve individual commercial and institutional buildings (MacCracken, 2003).
In a scenario of high penetration of renewables and increasing demand for cooling, ITES plays a crucial role. By simulating the costs of dispatch with and without ITES, its use may reduce energy costs by avoiding on-peak tariff time consumption (at the expense of energy losses in the storage cycle). When considered as a component of a renewables strategy, an electric utility can use surplus renewable energy that might occur during off-peak time, that otherwise would be dumped or sold at low prices. Moreover, ITES can effectively shift renewable energy generated during off-peak time to high demand periods when fossil generation is typically more costly, less efficient and more polluting (R.W. Beck, 2011).
The opportunity to design a ITES solution for Brazilian market originates from a strategy defined by the national energy regulation agency (ANEEL) which allows clients to adopt a time-of-use (ToU) tariff. Furthermore, starting in 2018, the white tariff allows users of low power costumers to adopt the option of paying different amounts depending on the time and day of the week. Figure 1.1 shows a comparison between white and conventional tariffs, exemplifying how the new option made available in 2018 works.
Figure 1.1 – Brazilian white tarrif
Conventional White tariff
Day hours Day hours
Conventional White tariff Ta ri ff T a ri ff
Business days Weekends
12 18 21
6 6 12 18
Adapted from ANEEL (web accessed. October of 2017)
As can be seen, during peak time, the tariff presents a considerable increase, while in off-peak the tariff is lower than conventional. For example, with this tariff strategy, clients in Salvador pay cheaper during the day and can pay up to eight times more during the peak time, between 18:00 and 21:00 (Coelba, 2017). The difference in the kWh paid between on-peak and off-peak time stands in other places such as Rio de Janeiro and Pernambuco (five times more). Therefore, an opportunity is seen by using cheap energy to produce cold and use it when the energy tariff is higher, providing an economy in electricity bill.
Thus, this project intends to explore this condition and evaluate a TES solution to be future introduced in the Brazilian market. In order to provide more focus in the conceptual and embodiment design phases, a previous product planning is performed, idealizing an ice battery
sufficient to supply 12.000 BTU/hour of refrigeration during 3 hours, with a total latent heat of 36.000 BTU stored. The object is to produce ice during off-peak time, storing it in an ice bank, and use it to cool during on-peak time, ensuring the performance needed to generate savings to a possible costumer. The refrigeration capacity was based on the duration of Brazilian peak-time tariff and LG™ catalog (2016), seeking for a modular product capable to adapt and cover as many split models as possible.
1.2 Objectives
This research aims to assess possible solutions of ITES technology for wider scale integration in Brazil’s commercial sector, to understand the technological aspects of the solution by developing a prototype and further modelling the system in LMS Imagine.Lab Amesim® 1D to uptake the new technology created. Thus, by performing a comparation between both physical and computational model and a further parametrization of models. Therefore, the specific objectives of this research are:
(1) Review literature from the available CTES technologies with similar global function;
(2) Identify the main design functions, search for working principles and develop a morphological matrix;
(3) Generate feasible concepts and compare them, selecting a winner concept using a qualitative evaluation criteria;
(4) Build and test a physical prototype, in order to prove the efficacy of the science;
(5) Develop a computational model of the most adequate concept in Amesim and simulate at prototype operation conditions, confirming and improving its feasibility;
(6) Compare both physical and computational models, performing a parametrization of the results;
1.3 Work Structure
This work is composed by six chapters, each providing information of a project development phase. Chapter 2 is a literature review,
presenting a background of the Product development process (PDP), modelling and simulation, air conditioning operation and cold thermal energy systems (CTES). Chapter 3 covers the methodological background and how the technical decisions were made, generating and selecting the concepts. Chapter 4 presents the development of the prototype, indicating the system assumptions to establish a comparation the equivalence between the computational and the physical models, Furthermore, it presents the model considerations and simplifications. Chapter 5 presents and discuss the results obtained by the computational model, assessing the behavior of the system. followed by conclusions and future works on Chapter 6.
2 LITERATURE REVIEW
2.1 Product Development Process
In a view of responsibilities involved in a product development process, including both technical and economic properties of a product, a design procedure is recommended to achieve good and optimized solutions, Pahl et al (2007).
This work is based on the design general phases of PRODIP (Integrated product development process), presented for the first time in Romano (2003) and adapted by Back et al (2008). The PRODIP methodology (Figure 2.1) provides support for companies to implement a systematic and formal procedure to execute the product development process, integrated to other business process, in order to reach innovative and viable solutions.
Figure 2.1 - PRODIP Methodology
Planning
Design
Implementation Informational Design Conceptual Design Preliminary Design Detailed Design Design especifications Product conception Product layout DocumentationAdapted from Romano (2003) and Back et al (2008) PRODIP design process is divided into four phases, such as: Informational design, conceptual design, preliminary design and detailed design. This work is focused specifically in the conceptual and preliminary design, seeking for the development of a concept and a techno-economic validation of the thermal storage system.
2.1.1 Conceptual Design
According to Back et al (2008), it is necessary to create a clear relationship between the problem inputs and outputs, in order to solve a technical issue. This is usually done by establishing the system general function and breaking into subfunctions or subsystems with lower complexity. The combination of these subfunctions provides the system functional structure. As a result, it facilitates the search for solutions.
Takács and Kamondi (2014) presents a function tree as an example of a function structure, as shown in Figure 2.2. Furthermore, the partial functions are broken in subfunctions which are yet broken in elementary functions 2 and 3. The main goal of this exercise is to determine tasks of the smallest parts of the structure, facilitating the main function interpretation by solving the problem from the bottom to the top of the function three.
Figure 2.2 – Function Tree
Main Function
Part Function 1 Part Function 2 Part Function 3 Part Function n
Sub-Function 1 Sub-Function 2 Sub-Function 3 Sub-Function 2 Sub-Function n
Element Function 1 Element Function 2 Element Function 3 Element Function n Element Function 2 Element Function 3
Adapted from Takács and Kamondi (2014)
Moreover, Pahl et al (2007) state that in case of original designs, the basis of the function structures refers to the requirements list of the problem. In this manner, it is possible to identify functional relationships and start the search for working principles which are going to be the physical solution to each subfunction.
One of the methods to organize and combine the working principles is Zwicky´s scheme (morphological matrix), presented for the first time in 1969 by Fritz Zwicky and widely used since then. Ritchey
(2002) explains that this method combines the subfunctions and the suitable principle of solution in rows and columns. For each function there are a number of possible solutions, which combined leads to a great number of possible combinations and consequently to a great number of feasible concepts. Figure 2.3 presents an example of how the morphological matrix can be applied to a certain problem. The functions of a main task are defined in the first column and each row is filled with a certain number of working principles. One example of this is the function “a.1 generate cold”, having four feasible solutions to fulfil the function. Therefore, by combining the principles of solution 2,1,2 for the functions a.1, a.2 and a.3 respectively, a design concept is created to execute the main task “Freeze PCM1”.
Figure 2.3 – Example of a combination of principles – Morphological matrix
Back et al (2008) argue that working principles can be originated from already existing process or from concept generation intuitive methods, such as brainstorming and analogy. In this way, the morphological matrix consists in a systematic approach, where is possible to combine different elements and parameters aiming a new solution for a problem (disruptive innovation). Nevertheless, it is recommended three or more working principles for each subfunction in order to enable a combination of solutions which leads to a good number of feasible
1 Phase Change Material (more detailed in Section 2.4) Working principles
designs. At the end, the combination results in design concepts to be further evaluated.
A method widely used to execute the process of concept selection is the Pugh Matrix (PM). Burge (2009) explains: “The PM is a type of matrix diagram that allows the comparison of a number of design candidates leading ultimately to which best meets a set of criteria”. Moreover, Back et al (2008) affirms the importance of the criteria definition and suggest fundamental aspects to be verified in the chosen criteria, such as:
• Compatibility with company capacity • Development risks
• User´s satisfaction • Financial aspects
• Innovation (differentiation to other products)
The Pugh Matrix is organized by choosing a baseline which is compared with the others for each criteria. Figure 2.4 brings an example where there are three alternative solutions, comparing by four criteria adopted.
Figure 2.4 – Example of a Pugh matrix
If the concept has a “+” in a criteria row, it is because the design is better than the baseline. Intuitively, the “-” at the box means that the design is worse than the baseline and the “0” means that there is no significant difference for that criteria. The result is obtained by summing the concept score. In this example, the thermomagnetic refrigeration
achieves a total score of 1(+), indicating to be a better choice for those criteria used. Also, a more sensible evaluation of concepts could be solved by adding extra levels of discrimination such as “++” (much better) and “--" (much worst) or doing another evaluating round changing the baseline concept.
2.1.2 Preliminary Design
The preliminary design starts from a chosen concept, originated from the combination of principles of solution, which at this point provides the source to work in an overall layout. At the end of this phase, the result must be a definitive layout.
According to Pahl et al (2007), the preliminary design differs from the concept design, because it involves a large number of corrective steps in which analysis and synthesis constantly alternate and complement each other. Moreover, at preliminary design phase, it is generally necessary several layouts so it can be evaluated simultaneously, in order to obtain information about advantages and disadvantages of each project variant. Therefore, a definitive layout will be selected if verified the best relation between function, durability, production, space, costs and other aspects of the project.
On the other hand, Chen (2015) brings a market-oriented product survey and affirms that mockups, prototypes and early product build to potential users helps to obtain a feedback on functionality, usability and pricing. At this approach, the development mode can be divided in three phase prototypes as shown in table 2.1.
Table 2.1 – The Development phases of a Hardware product
Phase 0 – Discovery and Feasibility
The objective is to prove the efficacy of the science and technology behind the idea by developing a “duct tape prototype”
Phase 1 – Engineering Prototype
Starting from the product definition in phase 0, the goal is to “bring it to life”. An integrated looks-like, work-like prototype with a design intended to production
Phase 2 – Engineering Verification (EV)
This phase is a design iteration, aiming the issues raised during previous tests of the engineering prototype. The EV form, function and finishing should represent the final product intent design.
Chen (2015) claims that the end of the phase 2 is the most critical gate review in the entire development project once is a crossover point, changing from design focus to a manufacturing/production focus. Therefore, the implementation of strategic prototypes at the preliminary design comes as an important condition to prove the technical feasibility and market acceptance.
In addition, Back et al (2008) states that preliminary layout modelling can be done both physical and computationally. Reasonably, the model characteristics can vary according to its relevance, level of detail and formality, always depending on the model purpose. Eventually, the product modelling can provide a simplified version of what is real, helping in project decisions with less risk and costs.
2.2 Modelling and simulation
Modelling and simulation techniques appeared as a way to recreate in a simplified mode what already exists. According to Back et al (2008), in the modelling process, a real object is represented by another abstract object, with simplifications, however presenting the same designation. The reason of a model creation is, therefore, to represent determined system, with less resources, reducing all characteristics of a real object to observe only key aspects.
Furthermore, models are classified by the approach, aiming in categorize and show the range of the task. Among the wide classification of models, this work focuses on analogic models, more specifically in simulation of multi-domain systems.
2.2.1 Dynamic simulation
Based on the presented scenario of modelling, this research seeks for a deeper application of technical systems modelling. Back et al (2008) describe advantages in performing computational simulations in industry, such as the reduction of physical prototypes, execution of more complex tests and reduction of the developing time.
There are a large number of software capable to simulate engineering systems by the signal language. According to Zrafi (2018), one of these tools is the Bond Graph Theory, largely accepted in engineering. Unlike block diagrams and signal-flow graphs, where the flow is unidirectional, bond graph theory is based on the cause and effect relationship of the energy transfer between the system variables. Gillet et al (2016) explains that each flow type element is followed by an effort type element and reciprocally. In thermodynamics, for example, the effort
variable is the temperature and the flow variable is the heat flow. In agreement with Borutzky (2010), bond graphs models represent physical process which energy is:
• Distributed
• Transferred from one power port to another • Transformed in the same energy domain
• Converted into another energy form, such as heat • Stored
This project uses LMS Imagine.Lab Amesim® 1D software, once it enhances the visual understanding of the system through a visual indication of the elements cause and effect relationship. Furthermore, the use of a virtual environment applied to multi-energy domains allows a mix of different existing libraries, such as air conditioning, two-phase-flow, thermos-hydro, mechanical and electrical libraries. However, Silva (2005) points that large systems usually demand blocks that do not exist in pre-built library and consequently there is a necessity to create “supercomponents”.
One example of an application in Amesim is shown in Figure 2.5. Note that the intention of the analysis presented is to compute mass flow and enthalpy increase as a function of rotary speed and pressure ratio of the compressor. Therefore, it is necessary to set inputs such as: hydraulic diameter of the pipe inlet (7), electric motor rotary speed (2,3), initial pressure and temperature in the tank and adiabatic chamber (6,8), initial state of the fluid just before the compression (5) and determine the fluid properties (1).
Figure 2.5 – System using a compressor
The mass flow and enthalpy variation can be affected not only by the rotary speed signal, but also by any other parameters mentioned before. As a result, it is possible to vary a great part of aspects of a system in order to optimize the design of determined system.
Although the existing benefits of the computational modelling, there are classic errors related to a poor application of any software in simulation. According to Back et al (2008), one of the methods to verify the accuracy of the model, independent of how the model was performed, is the sensibility analysis. This is done by verifying the outputs vary according to the model parameters change. If a slight change in a parameter result in a large variation of outputs, then the system is called sensible to this certain parameter. In other words, this means that a very precise measure of this parameter needs to be taken or even the model at its parameters needs to be revised. Therefore, a sensibility analysis is justified by not only to assess the iteration of the analysis factors, but also to evaluate the conformity between the model and the original project.
In conclusion, a deep knowledge of the problem is required to develop a good model, taking in consideration the physical phenomenon involved and having the ability to interpret and judge the results. In the present work, the knowledge about air conditioning operation and aspects of cold thermal energy storage are essential to the designer understanding and evaluation.
2.3 Air Conditioning
Buildings are designed to provide a secure and comfortable ambient, independent of weather conditions. There are many ways to keep pleasant the interior of buildings, such as electric fan, use of shading, solar orientation and other designs. Nevertheless, the most advanced and effective way to regulate temperatures to reach thermal comfort is air conditioning (AC), IEA2 (2018).
Today, available ACs have a large variation of models, since as a device capable to cool a single room to large scale systems. However, as mentioned in Chapter 1, a previous product planning established a project to meet the requirement of a 12.000BTU/hr mini-split system air conditioner.
Split systems in general are compound by a condenser, located outside the building and carries cold refrigerant through a pipe to an evaporator that is located inside the building (see Figure 2.6). More
specifically, the mini-split systems are called ductless due its short pipe length and present advantages such as lower distribution losses, increased energy efficiency and increased control of temperature in each room.
Figure 2.6 – Ductless mini split-system
LG® (web accessed. September of 2018)
Despite the ACs model variation throughout the world, all of them share the same technology, the vapor compression refrigeration cycle. According to Stoecker & Jabardo (2002), vapor compression refrigeration is based on a refrigerant fluid passing through physical processes, becoming liquid, superheated, satured and wet vapor. The refrigeration cycle uses the vaporization latent heat from a given refrigerant mass flow rate, to extract a large quantity of heat. The process can be better understood through the pressure-enthalpy or p-h diagram, as shown in Figure 2.7. The p-h diagram is a useful way to describe the liquid and gas phase of a substance. The saturation curve defines the boundary of the fluid states.
Figure 2.7 – Refrigeration cycle of a vapor compression system
Hundy et al (2008)
At the compression phase, the refrigerant comes from a satured state and gain pressure and consequently temperature, turning into a superheated gas. After that, there is a heat rejection at a constant pressure in the condensation phase, where the fluid slightly passes the saturation curve and becomes subcooled liquid. At this point, the fluid pass to a restriction of area, losing pressure abruptly which results in a drop of temperature and some flash off into vapor. The cycle ends the evaporating process, where the fluid absorbs heat from the ambient at constant pressure and become satured gas to be pumped again, restarting the cycle.
From the ratio of energy used and the energy moved, it is possible to measure the performance of the system. This ratio is termed the Coefficient of Performance (COP). Moreover, Hundy et al (2008) states that the COP of a vapor compression cycle is dependent on the properties of the refrigerant. Therefore, the type of refrigerant used in the AC system is also an important variable to be considered at this project development.
When it comes to HVAC/R³3industry, RSES (2005) affirms that R-22 has been the most used refrigerant despite environmental concerns incite the use of new refrigerants. Furthermore, Danfoss (2017) states that R22 (HCFC) is still the predominant refrigerant because of its wide applicability. The MMA4 (2011) clarifies that in Brazil, R22 and other fluids in the CFC family should be phased out in four years from 2015
3 Heating, Ventilation, Air-conditioning and Refrigeration 4 Ministério do Meio Ambiente - Ministry of Environment
due to the global phase-out of the CFCs known as the Montreal Protocol. However, Danfoss (2017) explains that while developed countries introduced HFCs in the 1990s to replace CFCs, emerging countries still find it difficult to replace them. Thus, HFCs are expected to dominate the Brazilian market in a few years. In addition, R-410a is pointed as the substitute of R-22 by the most major manufacturers.
The appendix A brings the P-h diagram of R-22 and R-410a cycles, putting in evidence their operation pressures. Comparing the evaporation phase of both cycles (process B-C), it can be seen that R-410a presents a low-pressure side of 141.2 psia against 87.7 psia in R-22 system (1.6 times bigger), resulting in a more complex and robust system. Thus, this work is directed to split-system ACs R-22 cycle, because its simplicity is seen as a better strategy in developing a new equipment.
Figure 2.8 illustrates the operating temperatures and pressures of a R-22 AC, considering a LG™ 12000BTU model at ASHRAE5 rated conditions.
Figure 2.8 – R-22 AC split-system at ASHRAE conditions
Condenser 54°C Evaporator 7.2°C Compressor Capillary tube Heat out Heat in Superheated vapour at 2200 kPa and 80°C Subcooled liquid at 2200 kPa
Liquid and vapour
at 550 kPa Dry saturated vapour
at 35°C
Adapted from LG (2014)
At the high-pressure side of the cycle, the condenser dissipates heat, dropping the temperature of the fluid from 80°C to 54°C, which makes the R-22 change from superheated vapor to subcooled liquid at 2200 kPa. After that, the expansion is caused by a capillary tube, where no heat is rejected or absorbed, but just a drastic reduction of pressure and consequently reduction of temperature from 54°C to 7.2°C. By this point, some of the fluid flashes off into vapor and the fluid enters into the
evaporator as a mixture. The cycle closes with the evaporator absorbing heat from the environment at 35°C and returns to the compressor to be pumped again.
Although ASHRAE rated conditions fixes an evaporating temperature for air conditioning in 7.2°C, LG (2014) considers as ACs normal operation zone evaporation temperatures between -10°C and 15°C.
2.4 Cold Thermal Energy Storage (CTES)
Energy storage plays a decisive role in the integration of renewable energy and flexibility of smart grids. According to ESMAP6 (2017), one important consideration in defining the potential of determined country is the stability of the electrical grid. In cases of unstable grid, customers can experience outages such as the occurred in Brazil in February of 2014 and January of 2015 (MME7, 2014, 2015). In addition, Arcuri et al (2016) states that Brazil experiences an increase of energy demand, especially because of the steady increase of air conditioning use.
Energy is defined by Oxford dictionaries (2018) as “the property of matter and radiation that is manifest as a capacity to perform work”. Therefore, energy does not limit itself to electric power but many other ways, such as thermal energy. Furthermore, cold thermal energy storage has been an object of study from a while, applied to large chiller-based systems and more recently in ice storage.
2.4.1 Sensible and latent heat systems
According to AHRI8 (2014), CTES systems can be classified as “latent” and “sensible”. The essential difference between these two groups is the use of the phase change of the material. Furthermore, Ismail (1998) affirms that latent systems has been preferably used due its high energy density and consequently less volume occupied by the phase change material. One example of this is a sensible thermal storage system using water as a way of storage (c=4.18J/°C.kg) against a latent thermal storage system using ice (L=333kJ/kg).
The latent heat efficiency is directly linked to the phase change material (PCM). Veerakumar & Sreekumar (2016) states that among the desirable properties, a PCM must have a high latent heat of fusion, high
6 Energy Sector Management Assistance Program 7 Ministry of Mines and Energy (Brazil)
density, present a melting temperature in the operation range and be cost effective. Table 2.2 shows some of the PCMs that could be used in CTES for AC application.
Table 2.2– PCMs feasible to AC application
PCM PC temperature (°C) Density (kg/m³) Latent Heat (kJ/kg)
s7 7 1700 150 A6 6 770 150 A4 4 766 200 A3 3 765 200 A2 2 765 200 E0 (water) 0 1000 332 E-2 -2 1070 306 E-3 -3.7 1060 312
Adapted from PlusICE® (2013)
Despite cold thermal energy storage be capable to use any material that store an amount of energy in its change of phase, water-ice have been dominating the application for HVAC systems. It becomes evident by analysing the table 2.2, where the PCM E0 (representing water) presents the highest latent heat.
2.4.2 ITES Technologies
Ice Thermal Energy Storage (ITES) is the application of water-ice as a PCM in a latent heat system. There are different arrangements to provide the cooling, varying how the heat transfer occurs in charge and discharge and how the ice is stored. Furthermore, AHRI (2014) classifies the ITES equipment in four types: Ice-on-coil with external melt, ice-on-coil with internal melt, encapsulated ice and unitary.
The ice-on-coil device consists of an evaporating refrigerant or a cold secondary coolant circulation through the coils, forming ice on the external surfaces during the charge cycle. The discharge cycle can be done with an external or internal melt. In the external melt process, water or a secondary coolant circulates through the tank, melting the ice formatted on the coils. In the internal melt process, a secondary coolant circulates thought the coil and is cooled while the ice formed external to the coil melts.
The encapsulated ice device consists of a tank packed with small containers in which water-ice or other PCM is encapsulated. During both charge and discharge process, a secondary coolant or water (if another PCM is being used) circulates through the tank, changing heat with the capsules, freezing or melting the storage medium.
Lastly, the unitary thermal storage systems (UTSS) consists of a heat exchanger submerged in a water filled tank. During the charge period, a cold refrigerant or secondary coolant passes through the heat exchanger, changing heat and freezing the tank. At the time of discharge, a fluid, which may be water, secondary coolant or a refrigerant pass through the heat exchanger inside the tank and melts the ice formed. The table 2.3 resume and show operation conditions of the existing types of ITES.
Table 2.3 – Classification and conditions of ITES technologies
Equipment Type
Charge Cycle Discharge Cycle
Charge fluid Period (h) Discharge fluid Entering fluid temperature (°C) Leaving fluid temperature (°C) Period (h) Ice-on-Coil (external melt) Secondary coolant 8 to 12 Water or secondary coolant 5 to 15 1.5 to 10 4 to 12 Refrigerant Ice-on-Coil (internal melt) Secondary coolant 8 to 12 Secondary coolant 5 to 15 3.5 to 10 4 to 12 Encapsulated Ice Water 6 to 12 Water 5 to 15 3.5 to 10 4 to 12 Secondary coolant Secondary coolant Unitary (UTSS) Refrigerant or Secondary coolant 6 to 14 Refrigerant, water or secondary coolant 3.5 to 15 3.5 to 10 4 to 12
Adapted from AHRI (2014)
The Icebear40, product presented by Ice Energy®, is one of the examples of ITES systems available in the US market for HVAC application, using the UTSS. In a study with a similar product, Willis (2010) states that the system can be installed in commercial DX (Direct Expansion) systems without change the annual AC energy consumption. The main objective of this equipment is to shift the load, avoiding peak demands for the grid. Figure 2.9 illustrates how a UTSS can be integrated to a packaged DX system.
Figure 2.9 – Hybrid cooling solution, combining a UTSS and a packaged DX system
Willis (2010)
It is possible to see that a “refrigeration management system” controls the flux of a secondary coolant to its charge and discharge cycle. Also, an ice make condensing unit is integrated to the system to provide the charge load, and an evaporator is integrated to the existing refrigeration system.
Despite of the potential of Brazilian market to energy conservation in AC systems, Arcuri et al (2016) states that currently there is only one manufacturer of ITES systems in Brazil, the Alpina CALMAC®. However, the solutions presented by CALMAC® are designed to attend large ACs, such as commercial buildings, shopping malls and supermarkets (20-1000TR9). Therefore, there is still a gap in Brazilian’s market to be attended by an energy storage solution capable to integrate most part of the ACs in the country.
As mentioned in Section 2.1, a disruptive design can be generated by a combination of working principles, especially because energy storage applied for small scale air conditioning remains an open field. Therefore, it is believed that a knowledge about existing CTES methods is an essential input in the seek for a new solution.
9 Ton of Refrigeration – One TR is approximately equivalent to 12,000
3 CONCEPT DESIGN METHODOLOGY
3.1 Establishing the Function Structure
In accordance with Chapter 1, this work intends to develop and validate a system capable to storage energy, supplying a 12,000BTU/hr AC during three hours. Therefore, it is needed to specify the project requirements to better understand how the system functions are organized. Firstly, it was specified the use of water as PCM due its better performance for AC application (Table 2.4) and availability of material. As mentioned before, despite the ASHRAE test conditions for split-system AC fixes an evaporation temperature in 7.2°C, it is acceptable to operate between -10°C and 15°C, validating therefore the evaporation temperature at 0°C (phase change temperature of water).
Furthermore, LG (2014) points that AC split-system operates with a COP between 3 and 3.3 at 35° ambient temperature, directly affecting system efficiency requirement. A system which does not operates near that COP would not represent real savings and benefits for its user. Expected durability and price also appear as essential aspects for energy storage system capable to provide interesting savings for a user.
Once defined the key aspects to be considered in the concept development, the methodology starts by establishing a global function.
3.1.1 Global Function
According to Dias et al. (2011), one method to obtain a function structure is a global function. From a global function, which is the main task the system will execute, it is possible to better understand the energy, signal and material involved in the process. Thus, Figure 3.1 presents the system global function, where the thicker line indicates the energy transformation, the middle line indicates the material transformation and the dotted line indicates signal transformation. The vertical lines indicate the external inputs.
Figure 3.1 – Global Function
As can be seen, “store thermal energy” represents the main function of an “ice battery”. Thus, there is energy transformation where is firstly considered electrical energy as energy input, necessary to remove heat from water. The material input considered is the PCM, which starts as water and becomes Ice. As a signal, it is possible to identify the material transformation by the alteration in the PCM volume. Since ice density is 8% lower than water, the ice formation can be measured by the water level, and thus the accumulated energy. In conclusion, the external elements are the ambient temperature inserting heat into the system and a refrigerant working as a vehicle for heat removal.
3.1.2 Function Tree
By understanding the basic functioning of the system in design, the global function can be easily divided in subfunctions, with less complexity. From a main function “store thermal energy”, it was identified the necessity of three subfunctions, which were further broken in elementary functions (Figure 3.2). The result is a function tree with three levels of functions.
Figure 3.2 - Function tree of an ice thermal storage system
Store thermal energy
Charge (Frost water)
Reduce heat exchange with ambient Discharge (Defrost ice) Absorb heat from water Rejects heat to
ambient Provide flow
Cool refrigerant fluid (Reject heat to ice) Generate cold
The first subfunction “charge” consists in the method utilized to remove heat from water. Therefore, it is divided in three elementary functions:
• Generate cold: The physical process involved in the process of generate cold;
• Absorb heat from water: The physical arrangement in which the heat can be absorbed from water;
• Rejects heat: The physical process responsible for dissipates the heat.
The second subfunction identified is the reduction of the heat exchange with the environment. Since there is a gradient between tank temperature (0°C) and ambient temperature (25°C), the heat flow tends to be from outside to inside the tank. Therefore, the reduction of heat exchange is an important point, regarding system efficiency.
In this case, the subfunction “reduce heat exchange with environment” has reached its lower level, being already the elementary function, in which will be determined a task for it. On the other hand, the last function, system discharge, is yet divided in two elementary functions:
• Provide flow: The mechanism responsible to provide flow to the refrigerant, in order to transport the heat; • Cool refrigerant: The physical arrangement in which the
heat is removed from the refrigerant, melting ice. As can be seen, the function tree provides the elements in which is possible to search and apply working principles for the design of the ice battery. However, as recommended by Back et al (2008), working principles can be generated by many methods, such as brainstorming and benchmarking. Therefore, the existing products through the world where analyzed and synthesized, in order to amplify the knowledge field.
3.1.3 Existing products / Benchmarking
As mentioned in Section 2.4, ITES systems has been used for over two decades, more specifically in large industrial cooling and air conditioning application. Although the ice battery in design be directed to small systems, it is believed that a benchmarking of the existing systems is a good start to map working principles. The products from CALMAC®, BAC®, Dunham Bush® and IceEnergy® were chosen due its similarity to the global function.
Figure 3.3 brings a comparison between the project decisions from the four companies according to the Function-means Law presented by
Chakrabarti (2002). Taking the function tree presented in Figure 3.2 as baseline, feasible means were suggested in order to identify how the function is performed. Each company configuration can be identified by a ball in the box of the used means.
Figure 3.3 – Function-means Tree of available ITES solutions
Store thermal energy for AC application
Charge (Frost PCM)
Reduce heat exchange with ambient Discharge (Defrost PCM) Absorb heat from PCM Rejects heat to ambient Polyurethane foam Direct contact Secondary heat exchanger Forced convection condenser Polystyrene foam
Provide flow Cool refrigerant fluid
(Reject heat to PCM) Glycol pump Refrigerant pump (R-410a) Glycol evaporator Additional heat exchanger Ice-on-Coil (external melt) Ice-on-Coil (internal melt) Encapsulated ice Unitary Glycol solution in coil Refrigerant (R-410a) in coil Generate cold Other refrigeration method Direct contact Secondary heat exchanger Natural convection Vapour compression cycle
- Ice Energy Icebear - BAC Ice Thermal Storage - Dunham Bush Ice-cel - Calmac Icebank
- Function
On the first function, unitary configuration is an unanimity among all companies. This can be explained by its simplicity in comparison with other arrangements. Moreover, analyzing the function “charge”, it is possible to see that IceEnergy has an innovative design, using R410a oil free as a refrigerant, justified by a gain in heat exchange efficiency. The use of glycol, on the other hand, is justified because of the less complexity and robustness of the system, once it operates with a simpler pump and at atmospheric pressure. Analogously, the “Discharge” function follows the same pattern, where BAC, Calmac and Dunham Bush use a glycol evaporator, while IceEnergy uses an external heat exchanger to defrost the ice.
Other important aspect of this analysis is the tendency of all companies to use vapor compression cycle as means for cold generation. However, none of them uses the expansion directly in contact with the PCM, having the necessity to include a heat exchanger to cool the refrigerant and then cool the water.
Following tasks clarification and how they are currently performed by existing products, the research can focus on the search for variant designs, in order to create a better solution for the determined application.
3.2 Concept development
As mentioned in Chapter 2, a concept can be developed by a combination of working principles referring to determined function. Once performed the brake of global function in lower levels, it is needed to fill a morphological matrix in order to provide a great possibility of combinations. After that, it will be possible to create concepts and possibly find a better solution.
3.2.1 Searching for Working Principles
The search for working principles is referred to the lower functions of the function tree, presented in table 3.1. Besides a benchmarking, the suggested working principles were based on internet search, brainstorming, refrigeration and air conditioning literature. Furthermore, it was established a target of four and a minimum of two working principles for each function.
Table 3.1 - Functions of ice thermal storage system
a) Freeze PCM (Charge mode)
(a.1) Generate cold (a.2) Absorb heat from PCM
(a.3) Reject heat to ambient b) Reduce battery energy
loss
(b.1) Reduce heat exchange with ambient
c) Melt PCM (Discharge mode)
(c.1) Provide coolant flow (c.2) Reject heat to PCM (a.1) Generate Cold:
According to Dossat (2004), there are three feasible means to absorb heat and generate cold for domestic use: Absorption refrigeration, thermoelectric refrigeration and vapor compression refrigeration. However, more recently studies suggest the concept of thermomagnetic refrigeration as strong alternative to conventional refrigeration systems. Therefore, a brief study for each refrigeration method was conducted, in order to better understand and evaluate its use for the ice battery system. Magnetic refrigeration is nowadays considered one of the most promising technologies for refrigeration, Mezaal et al (2017). Besides being an ecofriendly solution, it also operates consuming considerably less power than other technologies. However, researches are still looking to make this technology commercially feasible. Magnetic refrigeration is based on the magneto-caloric effect, which consists in a variation of temperature in ferromagnetic materials caused by a magnetic field induction. The phenomenon is an alignment of atoms as there is a magnetic momentum generated by the magnetic field,
As shown in figure 3.4, when the electromagnetic field is on, the alignment generates a constant temperature increase “∆T”. At this moment, it is possible to remove heat from the ferromagnetic material and return to its initial temperature. Then, by turning off the electromagnetic field, there is a “∆T” decrease in temperature, generating therefore cold. At this moment, it is possible to absorb heat by an evaporation process and restart the thermomagnetic cycle.
Figure 3.4 – Magnetic refrigeration process graph Magnetic field on Magnetic field off S N
Q
hQ
LT
T
S NAdapted from Mezaal et al (2017)
Sari & Balli (2014) affirms that proof of the concept for magnetic refrigeration is established and in the next years it can replace conventional systems. On the other hand, the vapor compression refrigeration has been dominant for the last two decades, due to present, in general, the best relation “efficiency x price”. Furthermore, vapor compression technology still remains an important area of research, especially in the development of new refrigerants. The seek for natural refrigerants has been driven by international agreements, such as the Montreal Protocol. The vapor compression refrigeration cycle is more detailed in Section 2.3 of this work.
According to Bansal & Martin (2000), absorption refrigeration technology becomes attractive when the energy source is abundant or cheap. One example is a system associated with industry where there is waste of heat. The absorption-refrigeration is based on laws of chemistry and physics, rather than mechanics and therefore works with no moving parts. As can be seen in Figure 3.5, a simple absorption refrigeration system is composed of a generator, separator, condenser, evaporator and absorber. The cycle starts by boiling the ammonia solution, which is composed by water, ammonia, hydrogen gas and sodium chromate. At the separator, the ammonia is distilled out of the solution and rise up to the condenser as a gas. Then, by dissipating heat, the pure liquid ammonia is combined with hydrogen gas before evaporation, causing a reaction which results in cold. At the evaporation, the ammonia solution absorbs heat and joins the mixture to restart the cycle.
