ABSTRACT
The caravan usage is implanted in the people’s way of living, for holidays, week-ends, and some elderly make them their home, a “moving home”. They seek for travelling and living in comfort, and the caravan is intended to provide well-being.
In many of the European Community counties there is a growing trend to seek for life in the countryside, valleys, and mountains, far from cities in a calm and pure atmosphere, despite their climate is not as mild as in the south western part of Europe, where Portugal is located. In order to obtain thermal comfort inside the caravan there are available several systems. In the present work it was selected a heat pump, a technology based on the vapour compression refrigeration systems. It produces energy for either heating or cooling, as function of the season, to promote thermal comfort. The focus was to study a convenient heat pump and its integration in caravans, due to space limitations.
Keywords: CFD, thermal comfort, caravan, heat pump.
INTRODUCTION
Nowadays the small caravans that people can afford, usually only have electrical heathers for heating purposes. The electricity used is bought in the campus they are sited. So the energy supply is higher than the real thermal needs of the caravan due to efficiency of the heater. In the warmer seasons, they are not provided with any type of cooling system and this can lead to inside overheating, and the thermal comfort of the users is not achieved.
In the research carried out it was analysed the possibility of using a heat pump either to reach thermal comfort in winter and summer time (Afonso, 2012). This equipment uses also electricity to run and is based on vapour compression refrigeration systems (Dinçer, 2003). As can be seen in Fig.1, it is the same equipment, with the same components, that satisfies the thermal loads of the caravan (house). It is only necessary to invert the fluid flow in the heat pump circuits, which is done internally (Martinez, 2005).
The caravan used in this study is shown in Fig.2.
It is composed by a bed room, a living space, kitchen and toilet, with a capacity for four persons.
Initially a survey was carried out to characterize the caravan’s structure, identifying its different elements, to determine their dimensions, Tab. 1, materials and thermal properties, Tab. 2.
The floor length is 4.4 m, the ceiling length is 3.7 m, the caravan’s width is 2.1 m and it’s height is 2.0 m.
Tab. 1 - Caravan’s walls and windows dimensions.
Walls Windows
Area (m2) Width (m) Area (m2) Width (m)
Front 3,69 0,024 0,78 0,003 Rear 3,98 0,024 0,25 0,003 Left 7,75 0,024 0,35 0,003 Right 7,50 0,024 0,60 0,003 Floor 9,29 0,043 - -Ceiling 7,81 0,030 -
-Tab. 2 - Caravan’s structure materials and thermal properties.
Thermal conductivity (W/(mK) ) Density (kg/m3) Specific heat (J/(kgK) )
Expanded Polystyrene 0,04 15 1210 Plywood 0,17 700 2000 Polyester 0,25 900 1700 Wood 0,17 700 2000 Aluminum 230,00 2700 870 Acrylic 0,22 1190 1600
The overall heat transfer coefficient, U, of each element was calculated by the following expression:
U= 1
R R R
Fig. 2 -The caravan studied: Caravelair Antares Luxe 400 caravan.
Fig. 1 - The working principle of heat pumps (internet site, 2016).
∑
i∑
Ri i ki
width and ki the thermal conductivity.
The overall heat transfer coefficients of the caravan’s structure are shown in Tab. 3, for the opaque elements, and in Tab. 4, for windows.
The heating and cooling loads were evaluated with the known properties of the structure of the caravan, as well with the measurements of the inside and outside temperatures. The temperature measurements were made with Testo data loggers, model T 175-H1. Since the caravan is not stationary, it was chosen various locations, including the most adverse climatic ones in Portugal, for typical Summer and Winter days.
The heating load considered the heat loss through the structure, ¿UA
(
Ti−Te)
, as well as the heat loss due to outdoor air infiltration, ¿m c´ p(
Ti−Te)
. The indoor air temperature was taken as 20.0 ºC.It was also verified that the caravan has several air leaks that contributes to thermal loads. They were measured using the tracer gas technique (Afonso, 2013). The air infiltration was measured by the gas tracer method with Bruel & Kjaer equipment, model Innova 1312 (former Brüel & Kjær, 2005)using SF6 gas.
The concentration of this gas in the air decayed with time, Fig. 3, and the slope of the linear regression line indicates the infiltration rate.
Tab. 4 - Overall heat transfer coefficients for caravan’s windows. Area (m2) U (W/(m2K) ) UA (W/K) Front 0,78 6,111 4,767 Rear 0,25 6,111 1,528 Left 0,35 6,111 2,139 Right 0,60 6,111 3,667
Tab. 3 - Overall heat transfer coefficients for caravan’s walls. Area (m2) U (W/(m2K) ) UA (W/K) Front 3,69 1,498 5,527 Rear 3,98 1,498 5,961 Left 7,75 1,498 11,608 Right 7,50 1,498 11,233 Floor 9,29 1,239 11,509 Ceiling 7,81 1,554 12,137
Fig. 3 - SF6 Concentration, c, in function of time.
As for the exterior temperatures, they vary with the locations chosen. For a perimeter of 100 km from Oporto (for week-end travels), Vila Real is the city with the lowest temperature, - 1.2 ºC, and as for the extreme climate, Penhas da Saúde was selected, with - 5.8 ºC. These temperatures have an occurrence probability of 2,5 %.
The coaling load considered the heat gain through the structure, due to opaque elements,
¿UA
(
Tsol−air−Te)
, Tsol-air is the sol-air temperature, and due to windows,Tsol−air−Ti
τI +U¿
¿A¿
, is the transmittance and I the incident solar radiation. It was also considered the heat gain due to outdoor air infiltration, ¿m c´ p
(
Te−Ti)
, as well as internal heat gains, 120 W for each occupant. The indoor air temperature was taken as 25.0 ºC.The solar radiation values were obtained from Solterm, a portuguese database with 308 meteorological reference years from LNEG (Laboratório Nacional de Energia e Geologia). As for the exterior temperatures, they vary with the locations chosen. For week-end travels, Vila Real is the city with the highest temperature, 33.0 ºC, and as for the extreme climate, Moura (Alentejo) was selected, with 37.0 ºC. These temperatures have an occurrence probability of 2,5 %.
Since the week-end travels will be more frequent, the heating and cooling loads for Vila Real were considered, respectively, 2.4 kW and 2.1 kW.
The heat pump selection followed. Market offers several systems with similar technical characteristics. Heating capacities ranges from 2.5 kW to 3.4 kW, and cooling capacities from 2.3 kW to 2.5 kW. The cop values for heating ranges from 3.6 to 4.6, and for cooling it ranges from 3.2 to 3.4.
For the system selected heating and cooling capacities are greater than the calculated heating and cooling loads, such that it operates at 80% full load. In this option besides the price and weight, the aerodynamic design and noise level were decisive.
The following stage involved CFD simulations, with Fluent Software (Fluent, 1998), to obtain the velocity and air temperature profiles of the inside air. The Gambit software was used to generate the three dimensional mesh with 128000 (80x40x40) hexahedron cells in a structure with (4m x 2m x 2m). The front of the caravan is directed towards the positive x axis, the
The heating and cooling loads are high due to the thermal properties of the constituent material of the caravan’s walls and roof, and because of the significant temperature differences between the exterior and the interior air.
Results show that the heating load for Vila Real is 2,4 kW, and for Penhas da Saúde it is 2,9 kW (extreme climate).
As for the cooling load, it was obtained 2.1 kW for Vila Real, and 2.6 kW for Moura (extreme climate).
For the system design it was considered the more frequent week-end travels, that is the heating and cooling loads for Vila Real.
For a typical Winter day, the heating cycle was analysed such that the indoor air temperature varied between 20.0 ºC and 25.0 ºC: at 20.0 ºC the system turned “on”, and at 25 ºC it turned “off”. Results show that the system was “on” for 180 s (3 min.), and “off” for 270 s (4.5 min.), being the heating cycle duration 450 s (7.5 min.).
Fig. 4 shows the air temperature (K) distribution 3 min. after turning the system “on”, and Fig. 5 shows the air velocity distribution at that time, in the longitudinal direction (A) and in the transversal direction (B).
For a typical Summer day, the cooling cycle was analysed such that the indoor air temperature varied between 25.0 ºC and 20.0 ºC: at 25.0 ºC the system turned “on”, and at 20 ºC it turned “off”. Results show that the system was “on” for 240 s (4 min.) , and “off” for 1200 s (20 min.) , being the cooling cycle duration 1440 s (24 min.).
Fig. 7 shows the air temperature (K) distribution 4 min. after turning the system “on”, and Fig. 8 shows the air velocity distribution at that time, in the longitudinal direction (A) and in the transversal direction (B).
Fig. 9 shows the air temperature (K) distribution 20 min. after turning the system “off”. It can be concluded from the results obtained for the indoor air temperature and velocity distribution that the heat pump technology is a good option to promote thermal comfort in caravans.
Fig. 6. Indoor air temperature (K) distribution for the heating cycle 4.5 min. after turning the system “off”.
Since this system runs on electric energy, the energy requirement was determined, based on the cop: heating cycle, COP = 2.4, and cooling cycle, COP = 2.1.
The electric generator selection followed. Market offers several systems with different technical characteristics and prices. Since the primary energy is of concern, among other reasons due to operating costs, three alternatives were taken: gasoline, diesel and GPL.
Three comfort scenarios (20.0 ºC < T < 25.0 ºC) were considered: 24 h comfort, 12 h comfort and 8 h comfort. An economic study showed that the operating costs are less than 1/10 of the costs if the electric energy is purchased from a camping park, for all scenarios.
The results show that the GPL generator is the best option. The return on investment is less than 2 years.
It can be concluded that an electric generator is economically a competitive solution towards the caravan’s energetic autonomy.
Fig. 9. Indoor air temperature (K) distribution for the cooling cycle 20 min. after turning the system “off”. ACKNOWLEDGMENTS
The authors gratefully acknowledge the collaboration of the staff in industry that provided the study object, the caravan.
REFERENCES
Afonso, C. F. A.; Termodinâmica para Engenharia, FEUP edições, 2012.
Afonso, C. F. A.; Tracer gas technique for measurement of air Infiltration and Natural Ventilation: Case Studies and New Devices for Measurement of Mechanical Air Ventilation in Ducts. International Journal of Low Carbon Technologies, 2013.
2016. http://www.savewithsrp.com/advice/graphics/heatpumpdiagram06.gif
Altener. “Photovoltaic Energy: technology manual, project and installations”, 2004. ASHRAE Handbook – Fundamentals. Atlanta. SI Edition, 2009.
Gambit (version: 2.3.16) and Fluent Manual (version: 6.3.26).
Dinçer, I, “Refrigeration Systems and Applications”, John Wiley & Sons, 1st Edition, 2003. Fluent User’s Guide, Version 4.0, Fluent Inc., Lebanon – NH, USA, 1998.
Instruction Manual “Photo Acoustic Multi-gas Analyzer; Model 1312” Brüel & Kjær.
Martínez, R., Javier, F., “Bombas ce callor y energías renovables en edificios”, International Thomson Editores Spain, 2005.
Solterm, portuguese database with 308 meteorological reference years from LNEG (Laboratório Nacional de Energia e Geologia).
