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Energy conservation and payback periods of large size solar water heater
Energy Convers. Mgmt Vol. 32, No. 4, pp. 371-374, 1991 Printed in Great Britain. All rights reserved
0196-8904/91 $3.00 +0.00 Copyright © 1991 Pergamon Press plc
ENERGY CONSERVATION A N D PAYBACK PERIODS OF LARGE SIZE SOLAR WATER HEATER N. M, NAHAR Division of Energy Management, Engineering and Product Processing, Central Arid Zone Research Institute, Jodhpur-342 003, India (Received 27 February 1990; receivedfor publication 12 December 1990)
Abstract--Hot water is an essential requirementin most agrobased industries. Firewood, coal, electricity and furnace oil are being used for obtaining hot water. These sources are not readily available and are scarce in most rural parts of Rajasthan. Fortunately, solar energy is plentiful which can be successfully harnessed for getting hot water. In this paper, performanceof the large size pressurizednatural circulation type solar water heater has been described. The heater can provide 750 I. of hot water at 80-85°C which can be retained at 70-75°C until the followingmorning when the tap water is 20-25°C. The economic analysis of the solar water heater has also been carried out. The relativelyshort payback periods show that the use of a solar water heater is very economicaland will conservea lot of conventionalfuels which are being wasted for merely obtaining hot water. Solar thermal energy
Solarwater heater
Paybackperiod
Energyconservation
INTRODUCTION Hot water is an essential requirement in most agrobased industries, e.g. oil mills, textiles etc. It is generally obtained by burning firewood, coal or furnace oil. Some of the industries obtain hot water by using electricity as well. The cutting of firewood causes deforestation which leads to desertification, and coal and furnace oil is brought from a large distance into Rajasthan. Electricity can be used for better purposes than heating water if a suitable alternative is provided. Fortunately, solar energy can play a vital role for heating water since technology for solar water heaters has already been developed and solar insolation is widely available [1]. The arid parts of India receive maximum radiation, i.e. 7600-8000 MJ/m 2 yr followed by semi-arid parts 7200-7600 MJ/m 2 yr and least on hilly areas where solar radiation is also appreciable, i.e. 6000 MJ/m 2 yr. Solar water heaters suitable for small agrobased industries are of the natural circulation type which has been developed and tested by Close [2], Gupta and Garg [3], Nahar [4], Norton et al. [5] and Nahar [6] and for large units are the forced circulation type developed by Garg [7] and Fenney and Klein [8]. In this paper, the performance of a large size natural circulation type solar water heater has been described, and its economic analysis has been carried out by considering the energy savings with respect to different fuels, annual compound interest, maintenance cost and inflation in fuel prices and maintenance cost.
D E S I G N AND P E R F O R M A N C E The solar water heater (Fig. 1) consists of an array of 10 flat-plate collectors, each having an absorber area of 1.9 m 2, and a double walled storage tank of 7501. capacity. The design of the flat-plate collector has been optimized in such a way that there is no wastage of raw materials, e.g. plain glass, mild steel sheet and aluminium sheet. The detail design of the solar water heater has been described by Nahar [6]. The performance of the solar water heater has been monitored extensively by taking the initial temperature of water in the tank in the morning, temperature attained in the evening and retained until the following morning. The heater can provide 7501. of hot water at an average temperature of 80-85°C in the evening which can be retained to 70-75°C until the following morning when the tap water temperature is 20-25°C. 371
372
NAHAR: LARGESIZE SOLAR WATER HEATER
Fig. 1. Solar water heater.
The efficiency of the solar water heater has been obtained by the following relation:
f
0 qu dO
0
n =
t'o
(1)
A J0 H d0 where A = flat-plate collector area (m2), H = global solar radiation incident on the collector surface (kCal/m2h), qu = rate of heat absorption by water (kCal/h), 0 = period of test (h), r / = efficiency. From equation (1), the efficiency of the solar water heater has been found to be 43%. ENERGY
SAVINGS
AND PAYBACK
PERIODS
The economic analysis of the solar water heater has been carried out by considering the equivalent savings in alternative fuels, viz. firewood, coal, furnace oil and electricity which are being used for heating water in agrobased industries. The heater saves 45,000 kCal or 52,335 Wh of fuel equivalent per day. As the heater is to be used in industry, it will be useful for about 300 days/yr. Accordingly, calculations have been made for the saving in different types of fuels and are shown in Table 1. The cash flows obtained by the heater have been obtained by taking into account 10% interest, 5% maintenance cost and 5% inflation in fuel prices and maintenance cost. The cost of the heater is Rs. 25,000/-(1.0 U.S. $ = 16.9 Rs.) which can be recovered in less than 2--4 yr when no tax incentives is considered. However, there is 100% depreciation allowed in the first year on solar water heaters, therefore, for profit making establishments, there will be a tax saving of Rs. 12,500/- in the first year in addition to the fuel saving. Therefore, cash flows have been worked out considering tax incentives also and are shown in Table 2. From Table 2, it is clear that the payback period of the heater reduces to less than 1-2 yr. The exact payback periods have also been calculated by the following relations, described by Brer [9], Duffle and Beckman [10]: n = {log ( E
-- M)/(a
-- b) -
log[(E -
M)/(a
-- b) -
c]}/log[(1 + a)/(1 + b)].
(2)
If the tax incentives is considered, then n = {log(E -
M)/(a
-- b) -
log[(E -
M)/(a
-
b) -
c ÷/]}/log[(1
+ a)/(1 + b)]
(3)
NAHAR:
LARGE SIZE SOLAR WATER HEATER
373
Table 1. Economic analysis of large size pressurized natural circulation type solar water heater (cost Rs. 25,000) Year
Cash flow (Rs.)
Interest (Rs.)
Maintenance (Rs.)
Energy saving (Rs.)
Net saving (Rs.)
2500.00 1560.73 468.07
1250.00 1312.50 1378.12
13,142.68 13,799.81 14,489.80
9392.68 10,926.58 12,643.61
2500.00 1573.32 497.25
1250.00 1312.50 1378.12
12,996.76 13,646.60 14,328.93
9246.76 10,760.78 12,453.56
1250.00 1312.50 1378.12 1447.03
9000.00 9450.00 9922.50 10,418.62
5250.00 6162.50 7185.63 8331.40
1250.00 1312.50
17,445.00 18,317.25
13,695.00 15,874,25
(a) Alternative fuel--firewood 0 1
2 3
-25,000.00 -15,607.32 -4680.74 +7962.87
(b) Alternative fuel--coal 0 1 2 3
-25,000.00 - 15,733.24 -4972.46 +748.11
(c) Alternative fuel--furnace oil 0 1 2 3 4
- 25,000.00 - 19,750.00 - 13,587.50 -6401.87 + 1929.53
2500.00 1975.00 1358.75 640.19
(d) Alternative fuel---electricity 0 1 2
- 25,000.00 - 11,305.00 +4569.25
2500.00 1130.50
Table 2. Economic analysis of large size pressurized natural circulation type solar water heater considering tax incentives (cost Rs. 25,000)
Year
Cash flow (Rs.)
Maintenance (Rs.)
Depreciation (Rs.)
Saving on tax (Rs.)
2500.00 310.73
1250.00 1312.50
25,000.00 --
12,500.00 --
13,142.68 13,799.81
21,892.68 12,176.58
2500.00 325.32
1250.00 1312.50
25,000.00
12,500.00
12,996.76 13,646.60
21,746.76 12,008.78
1250.00 1312.50
25,000.00
12,500,00
9000.00 9450.00
17,750.00 7412.50
1250.00
25,000.00
12,500.00
17,445.00
26,195.00
Interest (Rs.)
Energy saving (Rs.)
Net saving (Rs.)
(a) Alternative fuel--firewood 0 I 2
-25,000.00 -3107.32 +9069.26
Co) Alternative fuel--coal 0 1 2
- 25,000.00 -3253.24 +8755.54
(c) Alternative fuel--furnace oil 0 1 2
- 25,000.00 -7250.00 + 162.50
2500.00 725.00
(d) Alternative fuel---electricity 0 1
-25,000.00 + 1195.00
2500.00
Table 3. Payback periods of solar water heater Payback period (yr) Type of fuel
Without tax incentive
With tax incentive
Firewood Coal Furnace oil Electricity
2.39 2.42 3.78 1.85
1.16 I. 17 1.81 0.90
where a = interest rate per annum, b = inflation rate in energy and maintenance cost per annum, c = c o s t of solar water heater, E =energy saving per year, I = saving in income tax, m = maintenance cost per year and n = payback period. The payback period from equations (2) and (3) are computed for different fuels and are shown in Table 3. From Table 3, it is clear that payback periods are in increasing order with respect to fuel, electricity, firewood, coal and furnace oil.
374
NAHAR: LARGE SIZE SOLAR WATER HEATER CONCLUSION
The use o f solar water heaters will conserve a lot o f conventional fuels, viz. firewood, coal, furnace oil, electricity etc. which are presently being used for heating water in agrobased industries. The heater can supply hot water at 80-85°C which is generally required in most o f the agrobased industries, e.g. oil mills, textile processing, dairy, etc. The p a y b a c k periods suggest that the use o f solar water heaters is economical and will conserve a lot o f conventional fuels, e.g. firewood, coal, furnace oil, electricity, etc. Acknowledgements--The author is grateful to Dr J. Venkateswarlu, Director, CAZRI and Dr A. Ghanim, Head of Division
of Energy Management, Engineering and Product Processing for providing the necessary facilities and constant encouragement for the present study. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
IMD, Solar Radiation Atlas oflndia. India Meterological Department, New Delhi (1985). D. J. Close, Sol. Energy 6, 33 (1962). C. L. Gupta and H. P. Garg, Sol. Energy 12, 163 (1968). N. M. Nahar, Energy 9, 461 (1984). B. Norton, S. D. Probert and J. T. Gidney, Sol. Energy 39, 257 (1987). N. M. Nahar, Int. J. Ambient Energy 9, 149 (1988). H. P. Garg, Sol. Energy 14, 303 (1973). A. H. Fenney and S. A. Klein, Sol. Energy 40, 1 0988). K. W. B6er, Sol. Energy 20, 225 (1978). J. A. Duffle and W. A. Beckmann, Solar Engineering of Thermal Processes. Wiley, New York (1980).
0196-8904/91 $3.00 +0.00 Copyright © 1991 Pergamon Press plc
ENERGY CONSERVATION A N D PAYBACK PERIODS OF LARGE SIZE SOLAR WATER HEATER N. M, NAHAR Division of Energy Management, Engineering and Product Processing, Central Arid Zone Research Institute, Jodhpur-342 003, India (Received 27 February 1990; receivedfor publication 12 December 1990)
Abstract--Hot water is an essential requirementin most agrobased industries. Firewood, coal, electricity and furnace oil are being used for obtaining hot water. These sources are not readily available and are scarce in most rural parts of Rajasthan. Fortunately, solar energy is plentiful which can be successfully harnessed for getting hot water. In this paper, performanceof the large size pressurizednatural circulation type solar water heater has been described. The heater can provide 750 I. of hot water at 80-85°C which can be retained at 70-75°C until the followingmorning when the tap water is 20-25°C. The economic analysis of the solar water heater has also been carried out. The relativelyshort payback periods show that the use of a solar water heater is very economicaland will conservea lot of conventionalfuels which are being wasted for merely obtaining hot water. Solar thermal energy
Solarwater heater
Paybackperiod
Energyconservation
INTRODUCTION Hot water is an essential requirement in most agrobased industries, e.g. oil mills, textiles etc. It is generally obtained by burning firewood, coal or furnace oil. Some of the industries obtain hot water by using electricity as well. The cutting of firewood causes deforestation which leads to desertification, and coal and furnace oil is brought from a large distance into Rajasthan. Electricity can be used for better purposes than heating water if a suitable alternative is provided. Fortunately, solar energy can play a vital role for heating water since technology for solar water heaters has already been developed and solar insolation is widely available [1]. The arid parts of India receive maximum radiation, i.e. 7600-8000 MJ/m 2 yr followed by semi-arid parts 7200-7600 MJ/m 2 yr and least on hilly areas where solar radiation is also appreciable, i.e. 6000 MJ/m 2 yr. Solar water heaters suitable for small agrobased industries are of the natural circulation type which has been developed and tested by Close [2], Gupta and Garg [3], Nahar [4], Norton et al. [5] and Nahar [6] and for large units are the forced circulation type developed by Garg [7] and Fenney and Klein [8]. In this paper, the performance of a large size natural circulation type solar water heater has been described, and its economic analysis has been carried out by considering the energy savings with respect to different fuels, annual compound interest, maintenance cost and inflation in fuel prices and maintenance cost.
D E S I G N AND P E R F O R M A N C E The solar water heater (Fig. 1) consists of an array of 10 flat-plate collectors, each having an absorber area of 1.9 m 2, and a double walled storage tank of 7501. capacity. The design of the flat-plate collector has been optimized in such a way that there is no wastage of raw materials, e.g. plain glass, mild steel sheet and aluminium sheet. The detail design of the solar water heater has been described by Nahar [6]. The performance of the solar water heater has been monitored extensively by taking the initial temperature of water in the tank in the morning, temperature attained in the evening and retained until the following morning. The heater can provide 7501. of hot water at an average temperature of 80-85°C in the evening which can be retained to 70-75°C until the following morning when the tap water temperature is 20-25°C. 371
372
NAHAR: LARGESIZE SOLAR WATER HEATER
Fig. 1. Solar water heater.
The efficiency of the solar water heater has been obtained by the following relation:
f
0 qu dO
0
n =
t'o
(1)
A J0 H d0 where A = flat-plate collector area (m2), H = global solar radiation incident on the collector surface (kCal/m2h), qu = rate of heat absorption by water (kCal/h), 0 = period of test (h), r / = efficiency. From equation (1), the efficiency of the solar water heater has been found to be 43%. ENERGY
SAVINGS
AND PAYBACK
PERIODS
The economic analysis of the solar water heater has been carried out by considering the equivalent savings in alternative fuels, viz. firewood, coal, furnace oil and electricity which are being used for heating water in agrobased industries. The heater saves 45,000 kCal or 52,335 Wh of fuel equivalent per day. As the heater is to be used in industry, it will be useful for about 300 days/yr. Accordingly, calculations have been made for the saving in different types of fuels and are shown in Table 1. The cash flows obtained by the heater have been obtained by taking into account 10% interest, 5% maintenance cost and 5% inflation in fuel prices and maintenance cost. The cost of the heater is Rs. 25,000/-(1.0 U.S. $ = 16.9 Rs.) which can be recovered in less than 2--4 yr when no tax incentives is considered. However, there is 100% depreciation allowed in the first year on solar water heaters, therefore, for profit making establishments, there will be a tax saving of Rs. 12,500/- in the first year in addition to the fuel saving. Therefore, cash flows have been worked out considering tax incentives also and are shown in Table 2. From Table 2, it is clear that the payback period of the heater reduces to less than 1-2 yr. The exact payback periods have also been calculated by the following relations, described by Brer [9], Duffle and Beckman [10]: n = {log ( E
-- M)/(a
-- b) -
log[(E -
M)/(a
-- b) -
c]}/log[(1 + a)/(1 + b)].
(2)
If the tax incentives is considered, then n = {log(E -
M)/(a
-- b) -
log[(E -
M)/(a
-
b) -
c ÷/]}/log[(1
+ a)/(1 + b)]
(3)
NAHAR:
LARGE SIZE SOLAR WATER HEATER
373
Table 1. Economic analysis of large size pressurized natural circulation type solar water heater (cost Rs. 25,000) Year
Cash flow (Rs.)
Interest (Rs.)
Maintenance (Rs.)
Energy saving (Rs.)
Net saving (Rs.)
2500.00 1560.73 468.07
1250.00 1312.50 1378.12
13,142.68 13,799.81 14,489.80
9392.68 10,926.58 12,643.61
2500.00 1573.32 497.25
1250.00 1312.50 1378.12
12,996.76 13,646.60 14,328.93
9246.76 10,760.78 12,453.56
1250.00 1312.50 1378.12 1447.03
9000.00 9450.00 9922.50 10,418.62
5250.00 6162.50 7185.63 8331.40
1250.00 1312.50
17,445.00 18,317.25
13,695.00 15,874,25
(a) Alternative fuel--firewood 0 1
2 3
-25,000.00 -15,607.32 -4680.74 +7962.87
(b) Alternative fuel--coal 0 1 2 3
-25,000.00 - 15,733.24 -4972.46 +748.11
(c) Alternative fuel--furnace oil 0 1 2 3 4
- 25,000.00 - 19,750.00 - 13,587.50 -6401.87 + 1929.53
2500.00 1975.00 1358.75 640.19
(d) Alternative fuel---electricity 0 1 2
- 25,000.00 - 11,305.00 +4569.25
2500.00 1130.50
Table 2. Economic analysis of large size pressurized natural circulation type solar water heater considering tax incentives (cost Rs. 25,000)
Year
Cash flow (Rs.)
Maintenance (Rs.)
Depreciation (Rs.)
Saving on tax (Rs.)
2500.00 310.73
1250.00 1312.50
25,000.00 --
12,500.00 --
13,142.68 13,799.81
21,892.68 12,176.58
2500.00 325.32
1250.00 1312.50
25,000.00
12,500.00
12,996.76 13,646.60
21,746.76 12,008.78
1250.00 1312.50
25,000.00
12,500,00
9000.00 9450.00
17,750.00 7412.50
1250.00
25,000.00
12,500.00
17,445.00
26,195.00
Interest (Rs.)
Energy saving (Rs.)
Net saving (Rs.)
(a) Alternative fuel--firewood 0 I 2
-25,000.00 -3107.32 +9069.26
Co) Alternative fuel--coal 0 1 2
- 25,000.00 -3253.24 +8755.54
(c) Alternative fuel--furnace oil 0 1 2
- 25,000.00 -7250.00 + 162.50
2500.00 725.00
(d) Alternative fuel---electricity 0 1
-25,000.00 + 1195.00
2500.00
Table 3. Payback periods of solar water heater Payback period (yr) Type of fuel
Without tax incentive
With tax incentive
Firewood Coal Furnace oil Electricity
2.39 2.42 3.78 1.85
1.16 I. 17 1.81 0.90
where a = interest rate per annum, b = inflation rate in energy and maintenance cost per annum, c = c o s t of solar water heater, E =energy saving per year, I = saving in income tax, m = maintenance cost per year and n = payback period. The payback period from equations (2) and (3) are computed for different fuels and are shown in Table 3. From Table 3, it is clear that payback periods are in increasing order with respect to fuel, electricity, firewood, coal and furnace oil.
374
NAHAR: LARGE SIZE SOLAR WATER HEATER CONCLUSION
The use o f solar water heaters will conserve a lot o f conventional fuels, viz. firewood, coal, furnace oil, electricity etc. which are presently being used for heating water in agrobased industries. The heater can supply hot water at 80-85°C which is generally required in most o f the agrobased industries, e.g. oil mills, textile processing, dairy, etc. The p a y b a c k periods suggest that the use o f solar water heaters is economical and will conserve a lot o f conventional fuels, e.g. firewood, coal, furnace oil, electricity, etc. Acknowledgements--The author is grateful to Dr J. Venkateswarlu, Director, CAZRI and Dr A. Ghanim, Head of Division
of Energy Management, Engineering and Product Processing for providing the necessary facilities and constant encouragement for the present study. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
IMD, Solar Radiation Atlas oflndia. India Meterological Department, New Delhi (1985). D. J. Close, Sol. Energy 6, 33 (1962). C. L. Gupta and H. P. Garg, Sol. Energy 12, 163 (1968). N. M. Nahar, Energy 9, 461 (1984). B. Norton, S. D. Probert and J. T. Gidney, Sol. Energy 39, 257 (1987). N. M. Nahar, Int. J. Ambient Energy 9, 149 (1988). H. P. Garg, Sol. Energy 14, 303 (1973). A. H. Fenney and S. A. Klein, Sol. Energy 40, 1 0988). K. W. B6er, Sol. Energy 20, 225 (1978). J. A. Duffle and W. A. Beckmann, Solar Engineering of Thermal Processes. Wiley, New York (1980).