A Novel approach for Low temperature Condenser waste heat Utilization in winter air conditioning for overall Performance Improvement of a Power Plant in Northern India

A Novel approach for Low temperature

Condenser waste heat Utilization in winter

air conditioning for overall Performance

Improvement of a Power Plant in Northern

India

Rajendra Kumar Yadav

Research scholar Indira Gandhi National Open University 09717694845(M), [email protected]

Dr Ashish Agarwal

Associate Professor in the School of Engineering and Technology IGNOU,New Delhi, India

Dr Om Prakash Chourasia

Professor National Institute of technology Patna ,Bihar India Abstract:

The objective of this paper to highlight the scope of utilization of waste heat of condenser for winter air conditioning in and around thermal power stations. The vast amount of low grade condenser waste heat can be used to provide winter air conditioning by utilizing the existing system of year round central air conditioning without additional heavy capital expenditure .The present case study is about North India where peak winter lasts about 90 days . The waste heat utilization for winter conditioning will not only save large amount of electrical energy being used in electrical heaters but also generate additional revenue by selling out extra/spared power ,increase thermal energy utilization ,improve performance of the plant .The novel method of winter air conditioning will also reduce the heat load of the cooling towers , avoid pumping and blow down power as the returning water from air conditioning system can be used as blow down to maintain cooling water parameters in the plant.

Key words:

Air handling unit, Dry bulb temperature, Wet bulb temperature, cycle of concentration 1.Introduction :

Few years ago, air conditioning was considered as luxury , but nowadays it is necessity. This is due to rapid rise in standards of living, increased income through out the world .

Variation in average of Delhi wef to air conditoend temp(oC)

0 5 10 15 20 25 30 35 40

Janua ry

Mar ch

Ma y

July

Sep tem

ber

Nov em

ber

Months of the year

Te

m

e

p

ra

tur

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oC Average Temp oC

Airconditoned temp oC

Graph-1: periods of heating and cooling requirement for Delhi

2.0 Winter air conditioning:

In winter, In order to maintain the living space temperature at comfortable levels ,it is necessary to supply continuously a definite amount of heat in it. The amount of heat to be supplied is equal to the amount of the heat loss from the place .However there are some system requirement to transfer heat from a source to a space. Modern winter air conditioning systems need following for its functioning:

(i) A fluid for conveying heat.

(ii) A heat source to give the heat to working fluid

(iii)The arrangement of duct/piping to carry the working fluid.

(iv) The radiators /heat distribution arrangement for even and proper distribution of heat.

The working fluid may be warm air, hot water, low or high pressure steam. The advantage of warm air heating is that, it is relatively cheaper, simple in operation ,humidification of the system is easier compared with the systems using convector or radiators. The efficiency of air system is high compared to other systems as air is directly heated from the heat source and heat transfer takes place only once ,that to from flue gas to working air, the major quantity of the air is re circulated ,and air quantity circulation is regulated depending upon the weather conditions. The disadvantage of the system is limitation on applicability for large spaces, poor humidity control and improper air distribution.

The steam heating system is preferred in the large locations due to its high heat carrying capacity The heating in this type of system is due to condensation of steam in radiators/ heat distribution system. The advantages of steam heating system is its fast heating rate, easy control and lesser power consumption in pumping steam as compared to hot water system. However steam leakage causes unbearable noise levels, high temperature corrosion and lack of non utilization of waste heat. The electrical heating system is one of most modern method of winter air conditioning, where the air conditioned space is heated by electrical heaters. The system of electrical heating to space is very clean and noise free, However when the total energy involved to heat and achieve a particular temperature is considered , the Electrical heating system is least energy efficient. The Hot water heating system is other prominent space heating systems. The source of availability of hot water is the key factor for evaluation of its performance and Economics of operation . The cheaper Heat sources make the hot water heating system as one of the attractive and preferable methods. The waste heat sources are prominent candidate for the same and it is subject matter of this discussion, However potential of utilization of waste heat sources is affected by circuit efficiency [1] ,which is one of the important parameter for evaluation of the potential of heating from the sensible waste heat sources.

2.1. Winter Air Conditioning in North Indian climate:

upon the site conditions and availability of method and resources. In almost all north India, it is seen that in winter air conditioning, the general arrangement is to switch off the chiller system , pass the air through air duct after Air handling unit cooling coil .The forward duct of the Air handling unit includes rows of electrical heater and humiditiying nozzles. In winter only blower , heater and dumidityfiers remain in service. Winter conditions of North Indian like Delhi remains as under:

DBT WBT

%RH

Outside 2oC 0.0 73.00

Inside 22oC 15.78 50.00

It is seen from the graph-2 below that the average lowest temperature recorded in September month is only one day lower than temperature of conditioned air. The number of days of temperature lower than conditioned air temperature increases day by day in October month and it goes on till march. However the highest temperatures , which are generally in the day time are higher than required air conditioned temperatures in September ,October ,November month .It is higher or equal to required temperature in 25 days in December and highest recorded temperature remains lower in almost all days in January Month. This means that only few hours heating / air conditioning in October, November ,February ,March months is required ,however starting from 25 December up to end of January through out the day heating is required. Thus the temperature values which fall below the line of conditioned air temperature need to be raised to give comfort feelings to occupants.

High and Low Temeprature variation w ith respect to air conditoned temeprature (o_C)

0

5

10

15

20

25

30

35

40

1

3

5

7

9 11 13 15 17 19 21 23 25 27 29 31

Days

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oC

September High

September Low

October High

October Low

November High

November Low

November High

December

December Low

January High

January Low

February High

February Low

February High

March

March Low

Graph-1: Temperature variation with respect to conditioned air temperature

2.2 Hot water heating System:

high temperature water heating system is another method of space heating for winter air conditioning ,where in pressurized water at a temperature of around 250 0C is used to heat the establishments.

Thus hot water temperatures may be low, medium or high. The low and medium water temperatures can be sourced from waste heat sources of industries, which other wise remains unutilized. The condenser is one of the largest low temperature heat sources of any thermal power station as around 40 % of thermal energy of the thermal power plants is disposed off in the atmosphere to achieve condensation of steam , this remains totally unutilized . The table-1 below shows quantity of hot water flow and its temperature at one of the combined cycle power stations in the Month of January in North India. When the hot water temperature is compared with the ambient air temperature and amount of temperature to be raised to make the ambient air conditioned , it is appear ant that the warm water is useful heat source for winter air conditioning .

Table-1: Parameters of cooling water and ambient air

Day of the month January 2011

CW flow m3/hr

Condenser Inlet temperature

Condenser Outlet Temp

(oC) Ambient

DBT(oC)

Ambient Wet Bulb temperatu re (oC)

Required Air

conditioned air

Temperature (oC)

1 22523.3 26 34.7 20 18 22

2 22523.3 28 37.0 22 20.9 22

3 22388.4 29.3 38.7 22 21.0 22

4 22927.9 28.3 37.0 22 21.0 22

5 22927.9 26.3 36.7 20 19.0 22

6 22118.69 30.0 38.3 19 17.9 22

7 22590.0 26.3 35.0 20 19.2 22

8 21714.0 26.0 34.7 21 19.4 22

9 21848.0 28.3 37.0 21 20.0 22

10 22725.6 28.0 37.0 24 22.5 22

11 21646.4 27.0 37.0 21 19.4 22

12 21107.16 27.7 36.0 22 20.7 22

13 21174.6 30.0 36.3 21 17.2 22

14 21444.35 28.7 38.0 21 15.9 22

15 22253.57 27.0 36.8 23 20.0 22

16 22253.77 26.0 36.0 21 16.0 22

17 23265.07 26.7 36.7 20 17.9 22

18 23062.79 26.7 35.6 19 18.0 22

19 21579.22 26.0 35.0 23 19.5 22

20 22321.00 26.7 36.7 19 16.0 22

21 22860.48 28.0 37.0 20 14.5 22

22 22455.87 26.7 36.3 16 14.7 22

23 23265.09 27.7 34.3 17 16.0 22

24 23669.7 30.0 38.0 15 13.7 22

25 22253.57 26.0 35.0 15 13.8 22

26 22253.57 25.3 34.3 13.9 15 22

27 22658.57 29.0 37.0 19.0 20 22

28 23939.44 27.0 36.0 19.0 22 22

29 22590.75 28.0 38.2 19.2 22 22

30 23534.83 26.7 36.3 18.2 21 22

31 23332.52 26.3 36.3 20.0 21 22

The degree days for a month for a particular city is the sum of degree days for all degree days in the month. In European Countries , considerably lower inside temperature are maintained as compared to above, so the above linear relation is not applicable in these countries .In some European countries 60oF inside temperature is maintained and degree day is calculated accordingly. ThusDegree day is important parameter in evaluation of amount of heat required for heating . In the present case calculation of degree day gives estimation for the amount of energy and money saved in waste heat utilization for heating. Thus amount of heat to be provided inside the building to maintain thermal comfort in kWh is given by:

Q heating = Pspecific×24×3600*D/1000 [kWh]

Table-2: Effective Temperature rise of ambient air

Day of the month January 2005

Condenser Outlet

Temp (oC) Ambient DBT(oC)

Difference of cooling water and ambient air temeperat ure (oC)

Increase in air Temperature (oC)

For proposed air

conditioning with 38.24% circuit

Efficiency

Effective temperatu re (oC) of air after heating

Required Air

conditioned air

Temperature (oC )

1 34.7 20 14.7 5.62 25.62 22

2 37.0 22 15.0 5.70 27.7 22

3 38.7 22 16.7 6.39 28.39 22

4 37.0 22 15.0 5.70 27.7 22

5 36.7 20 16.7 6.39 23.39 22

6 38.3 19 19.3 7.38 26.38 22

7 35.0 20 15.0 5.70 25.7 22

8 34.7 21 14.7 5.62 26.62 22

9 37.0 21 16.0 6.11 27.11 22

10 37.0 24 13.0 4.97 28.97 22

11 37.0 21 16.0 6.12 27.12 22

12 36.0 22 14.0 5.35 27.35 22

13 36.3 21 15.3 5.85 26.85 22

14 38.0 21 17.0 6.50 27.5 22

15 36.8 23 13.8 5.28 28.28 22

16 36.0 21 14.0 5.35 26.35 22

17 36.7 20 16.7 6.39 2639 22

18 35.6 19 16.6 6.34 25.34 22

19 35.0 23 12.00 4.59 27.59 22

20 36.7 19 17.7 6.77 25.77 22

21 37.0 20 17.00 6.5 26.5 22

22 36.3 16 20.3 7.76 23.76 22

23 34.3 17 17.3 6.61 23.61 22

24 38.0 15 23.00 8.80 23.8 22

25 35.0 15 20.00 7.65 22.64 22

26 34.3 13.9 20.4 7.8 21.7 22

27 37.0 19.0 18.00 6.88 25.88 22

28 36.0 19.0 17.00 6.50 25.5 22

29 38.2 19.2 19.00 7.27 26.47 22

30 36.3 18.2 18.1 6.92 25.12 22

31 36.3 20.0 16.1 6.16 26.16 22

3.4 Winter air Air Conditioning arrangementin a thermal Power Plant :

Generally air conditioning in a Thermal power plant is provided through central /packed air conditioning systems. The central control room and other related systems have one centralized year round air conditioning system .The system under study consists of 80Tones of summer load capacity chilling system in facility building .The Winter heat load requirement is met with 24.51kW electrical heaters for facility building..

Figure-1:General arrangement of central year round air conditioning systems

The average change in chilled water temperature when the AHU is used for summer air conditioning is 9oF, water flow rates for O&M building is 243 USGPM . The water side pressure drop is 3.5MWG .The total electrical heating arrangement including all the heaters and pan humidity fier is 254 KW.

variation of air DBT and WBT with respect to requirement

0 10 20 30 40 50 60

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Days

T

e

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e

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p

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tu

re

(

o

C

),

%

R

H

Ambient DBT(oC)

Ambient Wet Bulb temperature

Air conditioned air Temperature(oC)

Conditoned air RH(%)

Graph-3: Comparison between ambient air and conditioned air

Cooling water Pump AHU water to air heat exchanger

AC chiller unit

Condenser

Refrigerant Compressor

Pump

Air flow to air duct

A comparison as per graph-3above between site conditions of power station conditioned air for the month of January shows that almost all part of the month when round the clock air heating for air conditioning is required, the variation between above is very small .This indicates that very small amount of heating is required to achieve the desired level. When all types of heat energy required running the above system in winter is calculated at plant level , it comes about 552.778KW. This includes Electrical load of Heaters, Pan humidifiers, Fan etc . However it is seen that the amount of heat addition and level of temperature increase required for winter air conditioning is very moderate and any source of heat, having 10-15oC higher temperature that ambient air temperature will be more than sufficient for winter air conditioning. is The present heat exchangers of air handling units have been designed to give a temperature difference of 9oF on both sides of working fluids. Therefore Low temperature cooling water can be source for heat for such system.

var iation of Conde ns e r cooling w ate r inle t and outle t te m pe rature and am bie nt air te m pe rature s

0

5

10

15

20

25

30

35

40

45

1

6

11 16 21 26 31

days

Te

m

p i

n o

C

cooling water inlet temp to condenser

cooling water outlet

temperature from

ocndenser Variation in cooling water temperature rise in

Condenser Ambient air DBT

Graph-4:Comparision of Cooling water temperature with ambient air

As from the graph-4 it is clear that cooling water temperature of the water coming out of the condenser during the period when heating is required is 15-18 oC higher than ambient air temperature .The daily variation in cooling water condenser outlet temperature in the months of November-2013,December-2013,January-2014 have negligible variation .

variation of air DBT and WBT wit hrespect to

requirement

0 10 20 30 40 50 60

1 4 7 10 13 16 19 22 25 28 31

Ambient air DBT

Ambient air WBT

conditioned air Temperature

Conditoned air RH(%)

Graph-5: conditioned and ambient air parameters

The hot water will flow through same lines in which chilled water flows at the time of summer air conditioning ,Since the chilled water pipes are insulated ,the temperature trop while hot water flows through these pipes will be negligible , However circuit efficiency of water carrying pipe has been taken as 93.56 %.The outlet pressure of cooling water coming out of condenser is around 1.9kg/cm2 , and the location of the high point of the AHU is around 20. 0 mtrs ,therefore including pressure drop of 3.5Mtr of water column ,the winter air conditioning is feasible with this arrangement.

Figure-2: Arrangement of novel approach for winter air conditioning at Thermal Power station Air

flow to air duct

Chilled from chillier unit

Hot cooling water from steam turbine condenser

Warm water to chillier unit

Cold cooling water to condenser return Cw header/to drain channel for Cooling tower blow down AHU water to air

heat exchanger

In this arrangement of winter air conditioning by waste heat of condenser .The cooling water coming out of condenser will be circulated in the same pipe lines, where through chilled water flows at the time of summer and monsoon air conditioning. The hot water will pass though the heat exchanger of the air handling unit and transfer its heat to the cold air ,which forces to pass through the heat exchanger in same way as it passes in the case of summer and monsoon season. As it is clear from the above diagram ,the cooling water out let from air handling unit can be further added to cooling water return line ,for its further cooling and uses in the cooling water circuit ,However if need arises ,to maintain cooling water chemical parameters ,the return water from air handling unit can be drained in the near by channel to reduce energy required to blow down water from cooling water basin.

5: Benefits and future scope:

The above novel approach of winter air conditioning results in waste heat utilization of condenser t ,the resultant energy saving in terms of saving in heaters, humidityfiers, blow down pumps ,cooling tower fans is 254kW. This is very attractive amount of electrical energy saving from a Thermal power station under study . The possibility of waste heat utilization and saving in electrical power is around 90days in a year, which is 25% of the period in a year, The equivalent saving @Rs4.85 per unit of electrical energy is Rs886968 per annum. The initial capacity cost in negligible and limited to investment in some mtrs of pipe length and valves etc., which is estimated as less than 2 lacs. This improves auxiliary consumption of the plant and more power is available from the power station for sale and generation of additional revenue. The amount of heat and quantity of water going to cooling tower is reduced , this reduces the cooling tower fan power to cool it down and pump used to blow down certain quantity of water for maintaining water parameters. The novel idea has further possibility of transporting hot water through insulated pipes to nearly located Commercial establishments to sale waste heat for cheaper and energy efficient winter air conditioning .

5. Conclusion:

This novel method of winter air conditioning at Thermal power station for facility building and near by establishments is excellent method of very large quantity of electrical energy conservation ,this can be used for a period of 25% of the time of the year in Northern Indian Climatic Conditions . The Method will result in over all green house gas reduction .The scheme if extended for commercial sale of heat to near by establishment for winter heat requirement will result in as additional resource of revenue for the power stations. References:

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