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Selection of micro turbines to meet electrical and thermal energy needs of residential buildings in Iran
Energy and Buildings 39 (2007) 1227–1234 www.elsevier.com/locate/enbuild
Selection of micro turbines to meet electrical and thermal energy needs of residential buildings in Iran M.A. Ehyaei, M.N. Bahadori 1,* School of Mechanical Engineering, Sharif University of Technology, Azadi Blvd., Tehran, Iran Received 20 August 2006; accepted 9 January 2007
Abstract Micro gas turbines are considered to meet the electrical, domestic hot water, heating and cooling energy needs of a residential building located in Tehran, Ahvaz and Hamedan. The building is 10 stories high and has a total of 8000 m2 floor area with the peak demands of electricity of 32.96 kW, DHW of 0.926 kW, heating load of 1590 kW and the cooling load of 2028 kW, when the building is located in Tehran. With these demands, 30 micro turbines of 30 kW (nominal power) are needed to meet all the energy needs of the building. The excess electricity generated by the micro turbines is to be used in a heat pump, and the energy in the exhaust gases is to be used to meet other thermal energy needs of the building. With proper energy conservation measures and the use of ceiling fans in each room, the peak heating and cooling demands of the building were reduced to 225 kW and 760 kW, respectively. With these measures, two micro gas turbines of 30 kW nominal capacity, or one of 40 kW, could meet all the electrical, DHW, heating and a great portion of the cooling needs of the building. The remaining cooling needs of the building during the hot hours of summer could be met by an additional absorption refrigeration, utilizing natural gas as its energy source. It is recommended that with energy conservation measures, the heating and cooling loads of buildings be reduced as much as possible, and micro gas turbines be employed to meet the electrical demands and a portion of heating and cooling needs. The remaining thermal energy needs are to be met through the use of natural gas. Only with these measures, the on-site combined heat and power (OS-CHP) is a viable option for residential buildings in Iran. # 2007 Elsevier B.V. All rights reserved. Keywords: Micro turbines; On-site CHP
1. Introduction Total electrical energy produced in Iran in 2004 amounted to 5760 TJ, corresponding with a per capita production of 96 MJ per year. The electricity was produced by the following systems: 55.4% by steam turbine power plants, 15.1% by gas turbine power plants, 22.7% by combined cycle power plants, 6.6% by hydroelectric power plants, and 0.2% by diesel engines [1]. There were a total of 167 GJ of electricity produced by wind and photovoltaic systems. This was about 0.0029% of the total electricity produced in the country. On the distribution of electricity to the consumers, about 8% of the energy was lost in the distribution lines [1]. The main sources of energy in Iran to meet heating, domestic hot water (DHW) and cooking are natural gas (in almost all * Corresponding author. Tel.: +98 21 227 38 261; fax: +98 21 660 00 021. E-mail addresses: [email protected] (M.A. Ehyaei), [email protected] (M.N. Bahadori). 1 ASME member. 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.01.006
cities) and kerosene. Cooling of buildings in summer is accomplished predominantly by evaporative coolers and vapor compression refrigeration. Absorption refrigeration is also employed in few large buildings, using natural gas to produce refrigeration. Summarizing, 93.4% of the electricity production, and 100% of the thermal energy requirements of buildings (for heating, domestic hot water and cooking) are met by employing fossil fuels, particularly natural gas. Almost all cooling energy requirements are met through electricity. It is estimated that the average conversion efficiency to meet the thermal energy needs of buildings is about 60%. The above figures show that almost 40% of the fossil fuels employed to meet the thermal energy requirements of buildings, and almost 75% of the fossil fuels, utilized to meet the electrical energy needs, are lost to the environment and are ‘‘wasted’’. There are several methods to reduce the thermal energy losses. One method is to employ combined cycles, another method is to utilize the heat rejected by the steam power plants for district
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Nomenclature COP e0 E N q0 Q ˙ Q Qc Qh T0 T1 T2 V˙ 1 V˙ 2
coefficient of performance electrical power needed (kW) power produced by each turbine (kW) number of micro turbine DHW energy needs (kW) energy in exhaust gases (kW) heating energy needs (kJ/day) cooling load of the building (kW) heating load of the building (kW) mean daily ambient air temperature (8C) temperature of the hot water used for bathing (8C) temperature of the hot water used for dish washing (8C) volumetric flow rate of the hot water used for bathing (l day1) volumetric flow rate of the hot water used for bathing (l day1)
Greek symbols a effectiveness of the heat exchanger babs COP of the absorption refrigeration bhp COP of the heat pump bref COP of the heat pump operating in refrigeration mode h turbine thermal efficiency (%)
heating and green houses (particularly in colder climates), in food industries, etc. A relatively new method to improve on the performance of the thermal energy systems is to employ the distributed power generation, or the on-site electricity production, with the rejected heat to be used to meet a portion or all of the thermal energy needs of the buildings. With advances made in recent years in the design, manufacture and operation of micro gas turbines, these units seem to be excellent candidates for on-site production of electricity, and the reduction of fossil fuel consumption for the thermal energy needs of buildings. This is particularly significant in Iran, where natural gas is already available for almost all buildings in large cities. Another advantage of the onsite combined heat and power production (OS-CHP) systems is their security. This is very important in an earth quake prone country like Iran. In case of earth quakes and other natural disasters, or acts of wars and terrorisms, damages done on the central power plants can result in large economic losses and discomfort to the people served by them. A broad research conducted by the authors on OS-CHP considers various systems such as micro gas turbines and IC engines, employing natural gas or bio-gas. Solar and wind energy are also considered for cities with favorable weather conditions. However, the present article deals with the employment of micro gas turbines (MGT), using natural gas in the cities of Tehran (with mild climate), Ahvaz (with hot climate) and Hamedan (with cold climate).
A detailed study considering the electrical and thermal energy needs of a residential building located in Tehran is carried out. Then the study is carried out for the same building, if it were located in the cities of Ahvaz and Hamedan. The use of micro gas turbines for OS-CHP, or on-site combined heat and power production, has been studied by several investigators [2–13]. 2. Estimation of the electrical energy needs of a residential building 2.1. Description of the building The building considered in this study is a 10-storey residential building with a total of 40 units, each with a floor area of 200 m2. The building has a height of 30 m, a length (in east-west direction) of 40 m, and a width of 20 m (in northsouth direction). The window areas are 30% of the areas of south and north walls and 20% of the areas of east and west walls of the building. The external and internal walls are 22 and 12 cm thick, respectively, all made of brick with gypsum plaster on the interior walls. The roof is also 22 cm thick, made of brick and roofing materials. No thermal insulation is employed in the walls or the roof of the building. 2.2. Measurement of the electrical energy needs of a residential unit To estimate the electrical energy needs of the building under consideration, it was decided to measure the actual electrical power consumption and the period of use of various electrical appliances and light bulbs, employed in a residential unit located in Tehran. The residential unit is occupied by an above average family of four adults. The electrical power consumption and the duration of use of the light bulbs and the electrical appliances in the residential unit were measured on 15 January. They were as follows: Light bulbs: 2 units, 236 W, 30 min, from 6 to 6:30 a.m., 3 units, 354 W, 30 min, from 6:30 to 7 a.m., 4 units, 472 W, 210 min, from 4:30 to 8 p.m., 3 units, 354 W, 180 min, from 8 to 11 p.m.; Television, 70 W, 300 min, from 4 to 9 p.m.; Computer, 121 W, 180 min, from 6 to 9 p.m.; Washing machine, 345 W, 90 min, from 9 to 10:30 a.m.; Hair dryer, 95 W, 60 min, from 7 to 7:30 a.m. and 8 to 8:30 p.m.; Iron, 964 W, 30 min, from 7 to 7:30 p.m.; Refrigerator, 205 W, 15 min per hour, all 24 h. For other days and months of the year, it was assumed that the 15th day of each month represented all days of that month. The electricity used for lighting was adjusted, considering the sun-set hour of that day [14], as compared with that measured for the residence on 15 January. Another adjustment was made for electrical energy consumed by the refrigerator. It was assumed that about 9 kg of fruits were
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Fig. 1. Ambient air dry-bulb and wet-bulb temperatures for Tehran, during the months of January, April and July.
placed in the refrigerator every week at the maximum daily ambient air temperature, as fruit shopping was normally done by the family in the afternoons, when the ambient air temperature is at its highest value. Fig. 1 shows the ambient air temperatures for Tehran, during the months of January, April and July. The total electrical energy consumption measured in a day in January and estimated for the other days and months of a year were compared with the electricity bill received by the family in each month of the past year. The agreement was very good, with the maximum deviation of 5%, occurring in July, and the minimum of 0.8%, occurring in December. The mean annual deviation was 2.34%. To estimate the total power and the electrical energy needs of the residential building under consideration, we assumed that all 40 residences were similar to the unit whose electrical energy consumption was measured, except that the use of the iron, computer, washing machine and the hair dryer was distributed in a 4-h span, each operating the same number of minutes given above. Figs. 2 and 3 show the total electrical
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Fig. 3. Total electrical power requirement of the residential building in a 24-h period, estimated for 15 July.
power requirement of the residential building in a 24-h period on 15 January, and 15 July, respectively. The areas under the power distribution curves in Figs. 2 and 3 represent the total electrical energy needs of the residential building on 15 January, and 15 July, respectively. These areas correspond with 867.6 MJ/day, for 15 January and 740.1 MJ/ day for 15 July. Fig. 4 shows the daily electrical energy needs of the building for various days of the year. It should be mentioned that Figs. 2–4 do not include the electrical power and energy needed to operate the electrical motors used for the central heating and cooling systems of the building. 3. Estimation of heating and cooling energy needs of the residential building located in Tehran Hourly heating and cooling energy needs of the building were estimated, employing the Carrier 2005 Hourly Analysis Program 4.2. 15th day of each month was assumed to represent all the days of that month. The energy needs were estimated for all the 4 units on the first floor, all the 4 units on the second to the ninth floor, and for the units located on the 10th floor. The weather conditions of Tehran, given in Fig. 1, were employed in the analysis. Fig. 5 shows the heating and cooling loads of the building (in kW) for 15 January, 15 April and 15 July, respectively. The total heating and cooling energy needs of the building (in GJ/day) are shown in Fig. 6. 4. Estimation of the energy needs for domestic hot water (DHW)
Fig. 2. Total electrical power requirement of the residential building in a 24-h period, estimated for 15 January.
In the residential unit occupied by four adults, the hot water consumption was measured as follows: The average amount of hot water used by four people for bathing was 358.5 l day1. The average hot water temperature found suitable by the occupants, was 41.5 8C. The average amount and the temperature of hot water used for dish washing were measured to be 30 l day1 and 52 8C, respectively. The cold water temperature was measured and was found to be about the mean
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Fig. 4. Daily electrical energy needs of the building for various days of the year.
daily ambient air temperature. The domestic hot water energy needs were then determined from: ˙ ¼ rw C w ½V˙ 1 ðT 1 T 0 Þ þ V˙ 2 ðT 2 T 0 Þ Q
(1)
˙ is the heating energy needs in kJ/day, rw and Cw the where Q water density in (kg/l) and specific heat in kJ/kg C, respectively, V˙ 1 and T1 the volumetric flow rate in l day1 and temperature (8C) of the hot water used for bathing and V˙ 2 and T2 are those for dish washing and T0 is the mean daily ambient air temperature. To determine the hourly energy needs for the domestic hot water consumed in the residential building of 40 units, we assumed that all units to have similar hot water consumption rate. We further assumed that the hot water consumption was uniformly distributed between 5 a.m. and 11 p.m., with no hot water consumed in the building between 11 p.m. and 5 a.m. Fig. 7 shows the daily energy needs of the building for domestic hot water.
Fig. 6. Heating and cooling energy needs of the building for various days of the year. The building is located in Tehran.
April and July. The daily heating and cooling and DHW needs on the 15th day of different months are shown in Figs. 5–7. It can be seen from Table 1 and Figs. 5–7 that the maximum electrical power requirement is 32.96 kW, occurring between the hours of 7 and 8 p.m. in July, maximum heating load is 1590 kW, occurring at 5 a.m. in January, the maximum cooling load is 2028 kW, occurring at 3 p.m. in July, and the maximum domestic hot water energy requirement is 0.926 kW, occurring between 5 a.m. and 11 p.m. in January. A micro turbine produces an electrical power of e, with the energy in the exhaust gases of q related to e by the following equation: 1 q¼e 1 h
(2)
Table 1 shows the electrical power and DHW requirements of the building at different hours of the 15th day of January,
where h is the turbine thermal efficiency. Both e and h are dependent on the ambient air pressure and temperature. In a given location with a nearly constant pressure, both e and h reduce as the ambient air temperature increases. A method to meet the energy needs of the residential building under consideration is to employ a number of micro
Fig. 5. Heating and cooling loads of the building, estimated for 15 January, 15 April and 15 July. The building is located in Tehran.
Fig. 7. Domestic hot water energy needs of the residential building.
5. A method for meeting the electrical, heating, cooling and domestic hot water energy needs of the residential building
M.A. Ehyaei, M.N. Bahadori / Energy and Buildings 39 (2007) 1227–1234 Table 1 Electrical power and DHW requirements of the building (in kW) Hour
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
15 January
15 April
15 July
Electricity
DHW
Electricity
DHW
Electricity
DHW
2.05 2.05 2.05 2.05 2.05 2.05 16.21 8.17 8.17 7.22 7.22 7.22 2.05 2.05 2.05 2.05 28.55 28.55 32.18 32.18 28.41 20.79 16.21 2.05
0 0 0 0 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0
2.41 2.41 2.41 2.41 2.41 2.41 2.41 8.54 8.54 7.59 7.59 7.59 2.41 2.41 2.41 2.41 10.00 28.91 32.54 32.54 28.77 21.15 16.57 2.41
0 0 0 0 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0
2.83 2.83 2.83 2.83 2.83 2.83 2.83 8.96 8.96 8.00 8.00 8.00 2.83 2.83 2.83 2.83 10.45 10.45 32.96 32.96 29.19 21.57 16.99 2.83
0 0 0 0 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0
The building is located in Tehran.
gas turbines to produce electricity to meet the electrical energy needs of the building, and to meet part of the heating and cooling energy needs through a heat pump. The energy in the exhaust gases is to meet the rest of the heating and cooling energy needs. We can use the following equation to determine the number of identical micro gas turbines needed during the heating season: ðne e0 Þbhp þ nqa q0 ¼ Qh
(3)
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Combining Eqs. (3) and (4) with Eq. (2), we have: 1 0 1 a q0 ¼ Qh ðne e Þbhp þ ne h 1 1 a q0 babs ¼ Qc ðne e0 Þbref þ ne h
(5)
(6)
6. Estimation of the number of micro gas turbines needed for the building located in Tehran We consider Capstone C30 Micro Turbines, with a nominal power of 30 kW. Variations, of e and h for this turbine with respect to the ambient air temperature are available in the manufacturer’s catalog. Fig. 8 shows these variations [15]. To use Eqs. (5) and (6) we assume the following properties for the system employed: bhp ¼ 3;
bref ¼ 2:5;
babs ¼ 0:7;
a ¼ 0:8
We first use Eq. (5) in order to find n for 15 January at 5 a.m. Referring to Table 1 and Figs. 1, 5 and 8, we obtain the following values: Ta = 2.8 8C, e0 = 2.05 kW, Qh = 1590 kW, q0 = 0.926 kW, e = 30 kW and h = 27%. With these values we find from Eq. (5), n = 10.31 or n = 11 units. For 15 January at hour 20 (8 p.m.) we have Ta = 10.5 8C, e0 = 32.18 kW, Qh = 865 kW, q0 = 0.926 kW, e = 30 kW and h = 26%. Then from Eq. (5) we obtain n = 6.08, or n = 6 units. For 15 July at hour 15, we have Ta = 39 8C, e0 = 2.83 kW, Qc = 2018 kW, q0 = 0.285 kW, e = 23.8 kW and h = 23%. From Eq. (6) we obtain n = 30.03 or n = 30. For 15 July at hour 19, we have Ta = 34 8C, e0 = 32.96 kW, Qc = 1136 kW, q0 = 0.285 kW, e = 24.8 kW and h = 23.5%. From Eq. (6) we obtain n = 11.37 or n = 12 units. It is obvious from the above calculations that the main task of the micro turbine in this case is to meet the cooling and heating energy needs of the residential building. With a total of 30 Capstone C30 micro turbines selected, all the energy needs of the building can be met at all the time.
where n is the number of identical micro gas turbines employed, e the electrical power produced by each turbine, e0 the electrical power needed, bhp the coefficient of performance (COP) of the heat pump employed, q the energy in the exhaust gases, a the effectiveness of the heat exchanger to utilize the energy to produce hot water or steam for heating, q0 the DHW energy needs, and Qh is the heating energy needs of the building. The unit for e, e0 , q, q0 and Qh is kW. All these energy terms are functions of time. For summer operation, or when cooling is needed, we can use the following equation: ðne e0 Þbref þ ðnqa q0 Þbabs ¼ Qc
(4)
where bref is the COP of the heat pump, operating in the refrigeration mode, babs the COP of the absorption refrigeration employed to produce chilled water for cooling, and Qc is the cooling load of the building.
Fig. 8. Variations of power and efficiency of Capstone C30 micro gas turbines with ambient air temperature.
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Depending on the energy needs throughout the year, a number of micro turbines can operate to meet both the electrical and thermal energy needs of the building. If we employ 10 micro gas turbines, from Eq. (5) and using the ambient air temperature of 3 8C, we obtain Qh = 1541.8 kW. Referring to Fig. 5 we notice that the heating energy needs of the building cannot be met between 2:30 and 7 a.m. This slight reduction of the supplied heat will cause a slight drop in indoor air temperature. This is generally tolerable at these hours by the occupants. For the same 10 micro turbines operating in July, and with the ambient air temperature of 33 8C, from Eq. (6) we obtain Qc = 989.45 kW. Referring to Fig. 5 we notice that the cooling provided by the combination of compression and absorption refrigeration systems can not meet the cooling energy needs of the building from 10:30 a.m. to 7:30 p.m. in July. With this arrangement, the indoor air temperature will rise to the extent that it may not be tolerated by the occupants. 7. Measures to reduce the heating and cooling energy requirements of the building We can take the following steps to reduce the cooling and heating energy needs of the building. (1) Employ ceiling fans in each room, when it is occupied. In this case the indoor air temperature of 28 8C can be selected (instead of 25 8C) and the cooling load of the building can be reduced from 2028 kW in July (see Fig. 5) to 1358.76 kW, a reduction of 33%. (2) Insulate the walls and roof of the building, use special sealants to reduce the cracks in windows, employ Persian Blinds on the outside of windows during the sunny hours in summer and winter nights, and use a curtain on the windows and the external walls during the summer days and winter nights. This curtain is high enough to touch the ceiling on top and the floor at the bottom. With these energy conservation measures it is possible to reduce the cooling energy needs of the building by 59% and the heating requirements by 86% [16]. With the energy conservation measures listed in items 1 and 2, it is possible to reduce the cooling load of the building to 760 kW. With this value for Qc, we find for 15 July at 3 p.m. (from Eq. (6)), n = 6.4 or 7 units. With the energy conservation measures listed in item 2, it is possible to reduce the heating load on 15 January at 5 a.m. to 225 kW. With this value for Qh, we find (from Eq. (5)), n = 1.5 or 2 units. It is seen from the above calculations, that with energy conservation measures, and the reduction of both cooling and heating loads, the number of micro turbines needed to meet all the energy needs of the building can be reduced appreciably. With the high cost of micro turbines, we suggest the employment of two Capstone C30 Micro turbines, with nominal power of 30 kW (or similar micro turbine manufactured by other companies), for the residential building under
Fig. 9. A schematic diagram of the micro gas turbines meeting the energy needs of the building in winter.
consideration, when the energy conservation measures listed above are taken in the building. With these micro gas turbines, all the electrical and heating energy needs, plus a portion of the cooling requirements of the building can be met. The rest of the cooling needs are to be met by an additional absorption refrigeration system, using natural gas as its energy source. Figs. 9 and 10 are schematic diagrams of the micro gas turbines operating in winter and summer, respectively. 8. Extension of the study to other cities in Iran Two cities of Ahvaz, with extremely hot climate, and Hamedan, with very cold climate, were selected and the studies made for Tehran were carried out for these cities. We considered the same building for analysis. Figs. 11 and 12 show the dry-bulb and wet-bulb temperature distributions
Fig. 10. A schematic diagram of the micro gas turbines meeting the energy needs of the building in summer.
M.A. Ehyaei, M.N. Bahadori / Energy and Buildings 39 (2007) 1227–1234
Fig. 11. Ambient air dry-bulb and wet-bulb temperatures for Ahvaz, during the months of January, April and July.
Fig. 12. Ambient air dry-bulb and wet-bulb temperatures for Hamedan, during the months of January, April and July.
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Fig. 14. Heating and cooling loads of the building, estimated for 15 January, 15 April and 15 July. The building is located in Hamedan.
Fig. 15. Heating and cooling energy needs of the building for various days of the year. The building is located in Ahvaz.
during the months of January, April and July for the cities of Ahvaz and Hamedan, respectively. Figs. 13 and 14 show the heating and cooling loads of the building located in these cities during the months mentioned. Finally, Figs. 15 and 16 show the heating and cooling energy needs of the building located in these cities for different days of the year.
The electricity needed for lighting and appliances and the energy for DHW are very close to the values obtained for the building located in Tehran. Following the same argument made for the use of micro gas turbines to meet the electrical and thermal energy needs of the building located in Tehran, we first need to reduce the heating and cooling loads of the building located in these
Fig. 13. Heating and cooling loads of the building, estimated for 15 January, 15 April and 15 July. The building is located in Ahvaz.
Fig. 16. Heating and cooling energy needs of the building for various days of the year. The building is located in Hamedan.
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cities to their minimum values and use micro gas turbines to meet the electrical energy needs of the building. The remaining electricity generated is to meet a portion of the heating and cooling needs through the use of heat pumps. The energy in the exhaust gases is to meet all or a portion of the thermal needs of the building, depending on the time of the day and day of the year. The remaining heating and cooling needs are to be taken care of by the use of piped natural gas, which is available in these cities. 9. Conclusions and recommendations Micro gas turbines were employed to meet the electrical and thermal energy requirements of a 10-storey residential building located in the cities of Tehran, Ahvaz and Hamedan. Natural gas, available in these cities through pipe lines, provides the energy source. The excess electrical energy produced by the micro gas turbines were employed in a heat pump/refrigeration system to meet a portion of the heating/cooling energy needs of the building. The energy in the gas turbine exhaust gases was used in a heat exchanger/absorption refrigeration system to meet the rest of heating/cooling and DHW needs of the building. Because of high heating and/or cooling loads of the building located in any of the cities considered, it is recommended that: (1) The heating and cooling loads of the building be reduced to their minimum values by proper energy conservation measures. (2) A minimum number of micro gas turbines be employed to meet just the electrical energy needs of the building. The excess electrical energy and the energy in the exhaust gases, be used to meet all or a portion of the thermal energy requirements of the building, depending on the building’s, needs. The rest of the building’s thermal needs should be met through the employment of appropriate systems, utilizing natural gas. For the building considered in this study and located in Tehran, and with a total of about 33 kW electrical power demand, this minimum number of micro
gas turbines correspond to one 40 kW or two 30 kW nominal-capacity micro gas turbines.
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Selection of micro turbines to meet electrical and thermal energy needs of residential buildings in Iran M.A. Ehyaei, M.N. Bahadori 1,* School of Mechanical Engineering, Sharif University of Technology, Azadi Blvd., Tehran, Iran Received 20 August 2006; accepted 9 January 2007
Abstract Micro gas turbines are considered to meet the electrical, domestic hot water, heating and cooling energy needs of a residential building located in Tehran, Ahvaz and Hamedan. The building is 10 stories high and has a total of 8000 m2 floor area with the peak demands of electricity of 32.96 kW, DHW of 0.926 kW, heating load of 1590 kW and the cooling load of 2028 kW, when the building is located in Tehran. With these demands, 30 micro turbines of 30 kW (nominal power) are needed to meet all the energy needs of the building. The excess electricity generated by the micro turbines is to be used in a heat pump, and the energy in the exhaust gases is to be used to meet other thermal energy needs of the building. With proper energy conservation measures and the use of ceiling fans in each room, the peak heating and cooling demands of the building were reduced to 225 kW and 760 kW, respectively. With these measures, two micro gas turbines of 30 kW nominal capacity, or one of 40 kW, could meet all the electrical, DHW, heating and a great portion of the cooling needs of the building. The remaining cooling needs of the building during the hot hours of summer could be met by an additional absorption refrigeration, utilizing natural gas as its energy source. It is recommended that with energy conservation measures, the heating and cooling loads of buildings be reduced as much as possible, and micro gas turbines be employed to meet the electrical demands and a portion of heating and cooling needs. The remaining thermal energy needs are to be met through the use of natural gas. Only with these measures, the on-site combined heat and power (OS-CHP) is a viable option for residential buildings in Iran. # 2007 Elsevier B.V. All rights reserved. Keywords: Micro turbines; On-site CHP
1. Introduction Total electrical energy produced in Iran in 2004 amounted to 5760 TJ, corresponding with a per capita production of 96 MJ per year. The electricity was produced by the following systems: 55.4% by steam turbine power plants, 15.1% by gas turbine power plants, 22.7% by combined cycle power plants, 6.6% by hydroelectric power plants, and 0.2% by diesel engines [1]. There were a total of 167 GJ of electricity produced by wind and photovoltaic systems. This was about 0.0029% of the total electricity produced in the country. On the distribution of electricity to the consumers, about 8% of the energy was lost in the distribution lines [1]. The main sources of energy in Iran to meet heating, domestic hot water (DHW) and cooking are natural gas (in almost all * Corresponding author. Tel.: +98 21 227 38 261; fax: +98 21 660 00 021. E-mail addresses: [email protected] (M.A. Ehyaei), [email protected] (M.N. Bahadori). 1 ASME member. 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.01.006
cities) and kerosene. Cooling of buildings in summer is accomplished predominantly by evaporative coolers and vapor compression refrigeration. Absorption refrigeration is also employed in few large buildings, using natural gas to produce refrigeration. Summarizing, 93.4% of the electricity production, and 100% of the thermal energy requirements of buildings (for heating, domestic hot water and cooking) are met by employing fossil fuels, particularly natural gas. Almost all cooling energy requirements are met through electricity. It is estimated that the average conversion efficiency to meet the thermal energy needs of buildings is about 60%. The above figures show that almost 40% of the fossil fuels employed to meet the thermal energy requirements of buildings, and almost 75% of the fossil fuels, utilized to meet the electrical energy needs, are lost to the environment and are ‘‘wasted’’. There are several methods to reduce the thermal energy losses. One method is to employ combined cycles, another method is to utilize the heat rejected by the steam power plants for district
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Nomenclature COP e0 E N q0 Q ˙ Q Qc Qh T0 T1 T2 V˙ 1 V˙ 2
coefficient of performance electrical power needed (kW) power produced by each turbine (kW) number of micro turbine DHW energy needs (kW) energy in exhaust gases (kW) heating energy needs (kJ/day) cooling load of the building (kW) heating load of the building (kW) mean daily ambient air temperature (8C) temperature of the hot water used for bathing (8C) temperature of the hot water used for dish washing (8C) volumetric flow rate of the hot water used for bathing (l day1) volumetric flow rate of the hot water used for bathing (l day1)
Greek symbols a effectiveness of the heat exchanger babs COP of the absorption refrigeration bhp COP of the heat pump bref COP of the heat pump operating in refrigeration mode h turbine thermal efficiency (%)
heating and green houses (particularly in colder climates), in food industries, etc. A relatively new method to improve on the performance of the thermal energy systems is to employ the distributed power generation, or the on-site electricity production, with the rejected heat to be used to meet a portion or all of the thermal energy needs of the buildings. With advances made in recent years in the design, manufacture and operation of micro gas turbines, these units seem to be excellent candidates for on-site production of electricity, and the reduction of fossil fuel consumption for the thermal energy needs of buildings. This is particularly significant in Iran, where natural gas is already available for almost all buildings in large cities. Another advantage of the onsite combined heat and power production (OS-CHP) systems is their security. This is very important in an earth quake prone country like Iran. In case of earth quakes and other natural disasters, or acts of wars and terrorisms, damages done on the central power plants can result in large economic losses and discomfort to the people served by them. A broad research conducted by the authors on OS-CHP considers various systems such as micro gas turbines and IC engines, employing natural gas or bio-gas. Solar and wind energy are also considered for cities with favorable weather conditions. However, the present article deals with the employment of micro gas turbines (MGT), using natural gas in the cities of Tehran (with mild climate), Ahvaz (with hot climate) and Hamedan (with cold climate).
A detailed study considering the electrical and thermal energy needs of a residential building located in Tehran is carried out. Then the study is carried out for the same building, if it were located in the cities of Ahvaz and Hamedan. The use of micro gas turbines for OS-CHP, or on-site combined heat and power production, has been studied by several investigators [2–13]. 2. Estimation of the electrical energy needs of a residential building 2.1. Description of the building The building considered in this study is a 10-storey residential building with a total of 40 units, each with a floor area of 200 m2. The building has a height of 30 m, a length (in east-west direction) of 40 m, and a width of 20 m (in northsouth direction). The window areas are 30% of the areas of south and north walls and 20% of the areas of east and west walls of the building. The external and internal walls are 22 and 12 cm thick, respectively, all made of brick with gypsum plaster on the interior walls. The roof is also 22 cm thick, made of brick and roofing materials. No thermal insulation is employed in the walls or the roof of the building. 2.2. Measurement of the electrical energy needs of a residential unit To estimate the electrical energy needs of the building under consideration, it was decided to measure the actual electrical power consumption and the period of use of various electrical appliances and light bulbs, employed in a residential unit located in Tehran. The residential unit is occupied by an above average family of four adults. The electrical power consumption and the duration of use of the light bulbs and the electrical appliances in the residential unit were measured on 15 January. They were as follows: Light bulbs: 2 units, 236 W, 30 min, from 6 to 6:30 a.m., 3 units, 354 W, 30 min, from 6:30 to 7 a.m., 4 units, 472 W, 210 min, from 4:30 to 8 p.m., 3 units, 354 W, 180 min, from 8 to 11 p.m.; Television, 70 W, 300 min, from 4 to 9 p.m.; Computer, 121 W, 180 min, from 6 to 9 p.m.; Washing machine, 345 W, 90 min, from 9 to 10:30 a.m.; Hair dryer, 95 W, 60 min, from 7 to 7:30 a.m. and 8 to 8:30 p.m.; Iron, 964 W, 30 min, from 7 to 7:30 p.m.; Refrigerator, 205 W, 15 min per hour, all 24 h. For other days and months of the year, it was assumed that the 15th day of each month represented all days of that month. The electricity used for lighting was adjusted, considering the sun-set hour of that day [14], as compared with that measured for the residence on 15 January. Another adjustment was made for electrical energy consumed by the refrigerator. It was assumed that about 9 kg of fruits were
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Fig. 1. Ambient air dry-bulb and wet-bulb temperatures for Tehran, during the months of January, April and July.
placed in the refrigerator every week at the maximum daily ambient air temperature, as fruit shopping was normally done by the family in the afternoons, when the ambient air temperature is at its highest value. Fig. 1 shows the ambient air temperatures for Tehran, during the months of January, April and July. The total electrical energy consumption measured in a day in January and estimated for the other days and months of a year were compared with the electricity bill received by the family in each month of the past year. The agreement was very good, with the maximum deviation of 5%, occurring in July, and the minimum of 0.8%, occurring in December. The mean annual deviation was 2.34%. To estimate the total power and the electrical energy needs of the residential building under consideration, we assumed that all 40 residences were similar to the unit whose electrical energy consumption was measured, except that the use of the iron, computer, washing machine and the hair dryer was distributed in a 4-h span, each operating the same number of minutes given above. Figs. 2 and 3 show the total electrical
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Fig. 3. Total electrical power requirement of the residential building in a 24-h period, estimated for 15 July.
power requirement of the residential building in a 24-h period on 15 January, and 15 July, respectively. The areas under the power distribution curves in Figs. 2 and 3 represent the total electrical energy needs of the residential building on 15 January, and 15 July, respectively. These areas correspond with 867.6 MJ/day, for 15 January and 740.1 MJ/ day for 15 July. Fig. 4 shows the daily electrical energy needs of the building for various days of the year. It should be mentioned that Figs. 2–4 do not include the electrical power and energy needed to operate the electrical motors used for the central heating and cooling systems of the building. 3. Estimation of heating and cooling energy needs of the residential building located in Tehran Hourly heating and cooling energy needs of the building were estimated, employing the Carrier 2005 Hourly Analysis Program 4.2. 15th day of each month was assumed to represent all the days of that month. The energy needs were estimated for all the 4 units on the first floor, all the 4 units on the second to the ninth floor, and for the units located on the 10th floor. The weather conditions of Tehran, given in Fig. 1, were employed in the analysis. Fig. 5 shows the heating and cooling loads of the building (in kW) for 15 January, 15 April and 15 July, respectively. The total heating and cooling energy needs of the building (in GJ/day) are shown in Fig. 6. 4. Estimation of the energy needs for domestic hot water (DHW)
Fig. 2. Total electrical power requirement of the residential building in a 24-h period, estimated for 15 January.
In the residential unit occupied by four adults, the hot water consumption was measured as follows: The average amount of hot water used by four people for bathing was 358.5 l day1. The average hot water temperature found suitable by the occupants, was 41.5 8C. The average amount and the temperature of hot water used for dish washing were measured to be 30 l day1 and 52 8C, respectively. The cold water temperature was measured and was found to be about the mean
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Fig. 4. Daily electrical energy needs of the building for various days of the year.
daily ambient air temperature. The domestic hot water energy needs were then determined from: ˙ ¼ rw C w ½V˙ 1 ðT 1 T 0 Þ þ V˙ 2 ðT 2 T 0 Þ Q
(1)
˙ is the heating energy needs in kJ/day, rw and Cw the where Q water density in (kg/l) and specific heat in kJ/kg C, respectively, V˙ 1 and T1 the volumetric flow rate in l day1 and temperature (8C) of the hot water used for bathing and V˙ 2 and T2 are those for dish washing and T0 is the mean daily ambient air temperature. To determine the hourly energy needs for the domestic hot water consumed in the residential building of 40 units, we assumed that all units to have similar hot water consumption rate. We further assumed that the hot water consumption was uniformly distributed between 5 a.m. and 11 p.m., with no hot water consumed in the building between 11 p.m. and 5 a.m. Fig. 7 shows the daily energy needs of the building for domestic hot water.
Fig. 6. Heating and cooling energy needs of the building for various days of the year. The building is located in Tehran.
April and July. The daily heating and cooling and DHW needs on the 15th day of different months are shown in Figs. 5–7. It can be seen from Table 1 and Figs. 5–7 that the maximum electrical power requirement is 32.96 kW, occurring between the hours of 7 and 8 p.m. in July, maximum heating load is 1590 kW, occurring at 5 a.m. in January, the maximum cooling load is 2028 kW, occurring at 3 p.m. in July, and the maximum domestic hot water energy requirement is 0.926 kW, occurring between 5 a.m. and 11 p.m. in January. A micro turbine produces an electrical power of e, with the energy in the exhaust gases of q related to e by the following equation: 1 q¼e 1 h
(2)
Table 1 shows the electrical power and DHW requirements of the building at different hours of the 15th day of January,
where h is the turbine thermal efficiency. Both e and h are dependent on the ambient air pressure and temperature. In a given location with a nearly constant pressure, both e and h reduce as the ambient air temperature increases. A method to meet the energy needs of the residential building under consideration is to employ a number of micro
Fig. 5. Heating and cooling loads of the building, estimated for 15 January, 15 April and 15 July. The building is located in Tehran.
Fig. 7. Domestic hot water energy needs of the residential building.
5. A method for meeting the electrical, heating, cooling and domestic hot water energy needs of the residential building
M.A. Ehyaei, M.N. Bahadori / Energy and Buildings 39 (2007) 1227–1234 Table 1 Electrical power and DHW requirements of the building (in kW) Hour
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
15 January
15 April
15 July
Electricity
DHW
Electricity
DHW
Electricity
DHW
2.05 2.05 2.05 2.05 2.05 2.05 16.21 8.17 8.17 7.22 7.22 7.22 2.05 2.05 2.05 2.05 28.55 28.55 32.18 32.18 28.41 20.79 16.21 2.05
0 0 0 0 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0.926 0
2.41 2.41 2.41 2.41 2.41 2.41 2.41 8.54 8.54 7.59 7.59 7.59 2.41 2.41 2.41 2.41 10.00 28.91 32.54 32.54 28.77 21.15 16.57 2.41
0 0 0 0 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0.606 0
2.83 2.83 2.83 2.83 2.83 2.83 2.83 8.96 8.96 8.00 8.00 8.00 2.83 2.83 2.83 2.83 10.45 10.45 32.96 32.96 29.19 21.57 16.99 2.83
0 0 0 0 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0.285 0
The building is located in Tehran.
gas turbines to produce electricity to meet the electrical energy needs of the building, and to meet part of the heating and cooling energy needs through a heat pump. The energy in the exhaust gases is to meet the rest of the heating and cooling energy needs. We can use the following equation to determine the number of identical micro gas turbines needed during the heating season: ðne e0 Þbhp þ nqa q0 ¼ Qh
(3)
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Combining Eqs. (3) and (4) with Eq. (2), we have: 1 0 1 a q0 ¼ Qh ðne e Þbhp þ ne h 1 1 a q0 babs ¼ Qc ðne e0 Þbref þ ne h
(5)
(6)
6. Estimation of the number of micro gas turbines needed for the building located in Tehran We consider Capstone C30 Micro Turbines, with a nominal power of 30 kW. Variations, of e and h for this turbine with respect to the ambient air temperature are available in the manufacturer’s catalog. Fig. 8 shows these variations [15]. To use Eqs. (5) and (6) we assume the following properties for the system employed: bhp ¼ 3;
bref ¼ 2:5;
babs ¼ 0:7;
a ¼ 0:8
We first use Eq. (5) in order to find n for 15 January at 5 a.m. Referring to Table 1 and Figs. 1, 5 and 8, we obtain the following values: Ta = 2.8 8C, e0 = 2.05 kW, Qh = 1590 kW, q0 = 0.926 kW, e = 30 kW and h = 27%. With these values we find from Eq. (5), n = 10.31 or n = 11 units. For 15 January at hour 20 (8 p.m.) we have Ta = 10.5 8C, e0 = 32.18 kW, Qh = 865 kW, q0 = 0.926 kW, e = 30 kW and h = 26%. Then from Eq. (5) we obtain n = 6.08, or n = 6 units. For 15 July at hour 15, we have Ta = 39 8C, e0 = 2.83 kW, Qc = 2018 kW, q0 = 0.285 kW, e = 23.8 kW and h = 23%. From Eq. (6) we obtain n = 30.03 or n = 30. For 15 July at hour 19, we have Ta = 34 8C, e0 = 32.96 kW, Qc = 1136 kW, q0 = 0.285 kW, e = 24.8 kW and h = 23.5%. From Eq. (6) we obtain n = 11.37 or n = 12 units. It is obvious from the above calculations that the main task of the micro turbine in this case is to meet the cooling and heating energy needs of the residential building. With a total of 30 Capstone C30 micro turbines selected, all the energy needs of the building can be met at all the time.
where n is the number of identical micro gas turbines employed, e the electrical power produced by each turbine, e0 the electrical power needed, bhp the coefficient of performance (COP) of the heat pump employed, q the energy in the exhaust gases, a the effectiveness of the heat exchanger to utilize the energy to produce hot water or steam for heating, q0 the DHW energy needs, and Qh is the heating energy needs of the building. The unit for e, e0 , q, q0 and Qh is kW. All these energy terms are functions of time. For summer operation, or when cooling is needed, we can use the following equation: ðne e0 Þbref þ ðnqa q0 Þbabs ¼ Qc
(4)
where bref is the COP of the heat pump, operating in the refrigeration mode, babs the COP of the absorption refrigeration employed to produce chilled water for cooling, and Qc is the cooling load of the building.
Fig. 8. Variations of power and efficiency of Capstone C30 micro gas turbines with ambient air temperature.
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Depending on the energy needs throughout the year, a number of micro turbines can operate to meet both the electrical and thermal energy needs of the building. If we employ 10 micro gas turbines, from Eq. (5) and using the ambient air temperature of 3 8C, we obtain Qh = 1541.8 kW. Referring to Fig. 5 we notice that the heating energy needs of the building cannot be met between 2:30 and 7 a.m. This slight reduction of the supplied heat will cause a slight drop in indoor air temperature. This is generally tolerable at these hours by the occupants. For the same 10 micro turbines operating in July, and with the ambient air temperature of 33 8C, from Eq. (6) we obtain Qc = 989.45 kW. Referring to Fig. 5 we notice that the cooling provided by the combination of compression and absorption refrigeration systems can not meet the cooling energy needs of the building from 10:30 a.m. to 7:30 p.m. in July. With this arrangement, the indoor air temperature will rise to the extent that it may not be tolerated by the occupants. 7. Measures to reduce the heating and cooling energy requirements of the building We can take the following steps to reduce the cooling and heating energy needs of the building. (1) Employ ceiling fans in each room, when it is occupied. In this case the indoor air temperature of 28 8C can be selected (instead of 25 8C) and the cooling load of the building can be reduced from 2028 kW in July (see Fig. 5) to 1358.76 kW, a reduction of 33%. (2) Insulate the walls and roof of the building, use special sealants to reduce the cracks in windows, employ Persian Blinds on the outside of windows during the sunny hours in summer and winter nights, and use a curtain on the windows and the external walls during the summer days and winter nights. This curtain is high enough to touch the ceiling on top and the floor at the bottom. With these energy conservation measures it is possible to reduce the cooling energy needs of the building by 59% and the heating requirements by 86% [16]. With the energy conservation measures listed in items 1 and 2, it is possible to reduce the cooling load of the building to 760 kW. With this value for Qc, we find for 15 July at 3 p.m. (from Eq. (6)), n = 6.4 or 7 units. With the energy conservation measures listed in item 2, it is possible to reduce the heating load on 15 January at 5 a.m. to 225 kW. With this value for Qh, we find (from Eq. (5)), n = 1.5 or 2 units. It is seen from the above calculations, that with energy conservation measures, and the reduction of both cooling and heating loads, the number of micro turbines needed to meet all the energy needs of the building can be reduced appreciably. With the high cost of micro turbines, we suggest the employment of two Capstone C30 Micro turbines, with nominal power of 30 kW (or similar micro turbine manufactured by other companies), for the residential building under
Fig. 9. A schematic diagram of the micro gas turbines meeting the energy needs of the building in winter.
consideration, when the energy conservation measures listed above are taken in the building. With these micro gas turbines, all the electrical and heating energy needs, plus a portion of the cooling requirements of the building can be met. The rest of the cooling needs are to be met by an additional absorption refrigeration system, using natural gas as its energy source. Figs. 9 and 10 are schematic diagrams of the micro gas turbines operating in winter and summer, respectively. 8. Extension of the study to other cities in Iran Two cities of Ahvaz, with extremely hot climate, and Hamedan, with very cold climate, were selected and the studies made for Tehran were carried out for these cities. We considered the same building for analysis. Figs. 11 and 12 show the dry-bulb and wet-bulb temperature distributions
Fig. 10. A schematic diagram of the micro gas turbines meeting the energy needs of the building in summer.
M.A. Ehyaei, M.N. Bahadori / Energy and Buildings 39 (2007) 1227–1234
Fig. 11. Ambient air dry-bulb and wet-bulb temperatures for Ahvaz, during the months of January, April and July.
Fig. 12. Ambient air dry-bulb and wet-bulb temperatures for Hamedan, during the months of January, April and July.
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Fig. 14. Heating and cooling loads of the building, estimated for 15 January, 15 April and 15 July. The building is located in Hamedan.
Fig. 15. Heating and cooling energy needs of the building for various days of the year. The building is located in Ahvaz.
during the months of January, April and July for the cities of Ahvaz and Hamedan, respectively. Figs. 13 and 14 show the heating and cooling loads of the building located in these cities during the months mentioned. Finally, Figs. 15 and 16 show the heating and cooling energy needs of the building located in these cities for different days of the year.
The electricity needed for lighting and appliances and the energy for DHW are very close to the values obtained for the building located in Tehran. Following the same argument made for the use of micro gas turbines to meet the electrical and thermal energy needs of the building located in Tehran, we first need to reduce the heating and cooling loads of the building located in these
Fig. 13. Heating and cooling loads of the building, estimated for 15 January, 15 April and 15 July. The building is located in Ahvaz.
Fig. 16. Heating and cooling energy needs of the building for various days of the year. The building is located in Hamedan.
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cities to their minimum values and use micro gas turbines to meet the electrical energy needs of the building. The remaining electricity generated is to meet a portion of the heating and cooling needs through the use of heat pumps. The energy in the exhaust gases is to meet all or a portion of the thermal needs of the building, depending on the time of the day and day of the year. The remaining heating and cooling needs are to be taken care of by the use of piped natural gas, which is available in these cities. 9. Conclusions and recommendations Micro gas turbines were employed to meet the electrical and thermal energy requirements of a 10-storey residential building located in the cities of Tehran, Ahvaz and Hamedan. Natural gas, available in these cities through pipe lines, provides the energy source. The excess electrical energy produced by the micro gas turbines were employed in a heat pump/refrigeration system to meet a portion of the heating/cooling energy needs of the building. The energy in the gas turbine exhaust gases was used in a heat exchanger/absorption refrigeration system to meet the rest of heating/cooling and DHW needs of the building. Because of high heating and/or cooling loads of the building located in any of the cities considered, it is recommended that: (1) The heating and cooling loads of the building be reduced to their minimum values by proper energy conservation measures. (2) A minimum number of micro gas turbines be employed to meet just the electrical energy needs of the building. The excess electrical energy and the energy in the exhaust gases, be used to meet all or a portion of the thermal energy requirements of the building, depending on the building’s, needs. The rest of the building’s thermal needs should be met through the employment of appropriate systems, utilizing natural gas. For the building considered in this study and located in Tehran, and with a total of about 33 kW electrical power demand, this minimum number of micro
gas turbines correspond to one 40 kW or two 30 kW nominal-capacity micro gas turbines.
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