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Conception of combination of gas-engine-driven heat pump and water-loop heat pump system
International Journal of Refrigeration 28 (2005) 810–819 www.elsevier.com/locate/ijrefrig
Conception of combination of gas-engine-driven heat pump and water-loop heat pump system Zhiwei Liana,*, Seong-ryong Parkb, Wei Huangc, Young-jin Baikb, Ye Yaoa a b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China Department of Energy Efficiency, Korea Institute of Energy Research, Taejeon 305-343, South Korea c Shanxi Building Design and Research Institute, Shanxi 710001, China Received 6 October 2003; received in revised form 31 December 2004; accepted 28 February 2005 Available online 26 May 2005
Abstract Conception of combination of gas-engine-driven heat pump (GHP) and water-loop heat pump system (WLHPS) is firstly presented in the paper in order to reduce the energy consumption of air conditioning system further. Design of the new system is introduced through an actual project in China and compared with a conventional air-conditioning system (CACS) and conventional WLHPS (EHP-WLHPS) in terms of technical characteristics and payback period. It is found that the payback period of GHP-WLHPS is about 2 years when compared with CACS and 2.6 years with EHP-WLHPS on the average. So it is worth being more widely applied. And several barrier to application are also discussed. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Air conditioning; China; Heat pump; Gas engine; Modelling; Hydraulic circuit; Comparison; Payback
Conception d’une pompe a` chaleur entraıˆne´e par un moteur a` gaz relie´e a` un syste`me de pompe a` chaleur a` boucle d’eau Mots cle´s: Conditionnement d’air ; Chine ; Pompe a` chaleur ; Moteur a` gaz ; Mode´lisation ; Circuit hydraulique ; Comparaison ; Amortissement e´conomique
1. Introduction It is well known that more and more energy is being consumed in air conditioning system in almost every country in the world. And much attention has been paid on it. Heat pump is known as a kind of energy saving air conditioning
* Corresponding author. Tel.: C86 21 62933240; fax: C86 21 62932601. E-mail address: [email protected] (Z. Lian).
0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.02.004
equipment, which is mostly driven by electricity. Current practice is mainly to convert fuel to electric power at a central power plant and reject the waste heat to the environment. The electricity is then transmitted to the heat pump motor, which provides the work input to the heat pump cycle. As is seen from Fig. 1, significant losses will occur from fuels to work of the heat pumps in such a case [1]. An obvious way to improve overall fuel utilization is to locate the fuel conversion process closer to, where the exhaust heat can be productively used. A combustion engine directly driving a heat pump compressor has about the same
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Nomenclature a b c CACS COP COPh EHP Ep1 Ep2 Es1 Es2 GHP m Qc Qc,h
number of GHP packages running in cooling mode number of GHP packages running in heating mode specific heat of water in the loop, kJ/kg 8C conventional air conditioning system co-efficient of cooling performance of GHP co-efficient of heating performance of GHP electric-driven heat pump first year or annual energy cost of CACS, $ first year or annual energy cost of GHPWLHPS or EHP-WLHPS, $ initial cost of CACS, $ initial cost of GHP-WLHPS or EHP-WLHPS, $ gas-engine-driven heat pump payback period, year heat quantity rejected to the water-loop by GHP unit in cooling mode, kJ heat quantity provided by GHP unit in heating mode, kJ
overall fuel-to-heat or cooling efficiency as an electric unit. An additional advantage is that the combustion engine’s waste heat can be utilized at the site to provide supplemental space heating or water heating. Therefore, gas-enginedriven heat pump (GHP) will have a high efficiency of fuel utilization. At this time no more than 1/3 of energy consumed in a conventional heat pump is needed in GHP to achieve the same heating effect (Fig. 2) [1]. This improved utilization of fuel is one of the most important factors that have given rise to the development of unitary GHP package in Japan, Europe, and the USA [2–5]. And the development of the GHP package in Japan was also the result of regulation for reducing the peak load in the electric grid. Just like the air-cooled heat pumps, all the GHP packages now are of air-cooled type and some energy is still wasted when heat is transferred between external air and the heat exchanger inside the outdoor unit, especially when there is different requirements of cold and heat inside a building. Water-loop heat pump system (WLHPS), which
Qe
heat absorbed by GHP unit in heating mode from the water-loop, kJ Qe,c cooling quantity provided by GHP unit in cooling mode, kJ Qex heat quantity recovered from the exhaust gas of GHP, kJ Qfuel heat quantity produced by fuel supplied to GHP, kJ SCOP whole system’s performance coefficient of GHP-WLHPS Wboil energy consumed by boiler in GHP-WLHPS, kJ Wtower energy consumed by cooling tower in GHPWLHPS, kJ WLHPS water-loop heat pump system V volume of water in WLHPS, m3 r density of water in the loop, kg/m3 DQ heat quantity transferred from GHP units to water-loop, kJ Dt change of temperature of the water in WLHPS, 8C
has the function of heat recovery by transferring the heat in the inner zone (core area) to the outer zone (perimeter area) by the circulation water, makes it possible to use this part of energy [6]. Therefore, this paper presents a new conception of combination of GHP and WLHPS in order to save energy further in air conditioning systems.
2. Conception of combination of GHP and WLHPS Although GHP was appeared on the Japanese HVAC marketplace in 1986 and WLHPS even as early as 1960s in the USA, the combination of GHP and WLHPS is completely a new conception of their application. So some discussions on the new system are presented in the paper. 2.1. Description of the new system As to a GHP package, its typical system is split into
Fig. 1. Losses of conversion process from fuels to work of a heat pump.
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Fig. 2. Comparison of energy conversion process between conventional EHP and GHP.
2 parts: one outdoor unit and one (or several) indoor one(s). A schematic view of the outdoor unit of a water-cooled GHP package, operating in heating mode, is shown in Fig. 3(a). In the case of heating mode, the refrigerant-water heat exchanger in Fig. 3(a) works as an ‘evaporator’ that absorbs heat from the water in the water-loop. While in the case of cooling mode, it works as a ‘condenser’ that rejects heat to the water in the water-loop. It can be seen that waste heat is recovered both from the engine jacket and from the exhaust by means of a compact heat exchanger. Those parts of heat can be transferred to heat the water in the water-loop in order to improve the performance of WLHPS in winter or to be used for hot water supply in a whole year. And besides, the heating performance of the GHP system can be enhanced by raising the lower pressure of the refrigerant in winter and the efficiency of the gas engine in the GHP package will also be improved because the heat quantity of the exhaust gas decreases after the heat recovery.
Fig. 3(b) shows the schematic view of WLHPS, where the GHP packages can be applied, among them connecting points are A and B, respectively, in Fig. 3(a) and (b). Much more energy can be saved especially when they are applied to such a building with inner zone and outer zones, where some of the GHP packages work in cooling mode while some work in heating mode. At this time, the heat rejected from the GHP packages work in refrigeration mode inside the building could be transferred to the outer zone by the circulation water in the water-loop and heat recovery could be realized. 2.2. Performance model of the new system To explain the performance of the GHP-WLHPS, its performance model is discussed in three cases as ‘all cooling’, ‘all heating’, and ‘part cooling and part heating’, respectively.
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DQ Z
813
a aCb b X X X ðQc Þk C ðQex Þn K ðQe Þl kZ1
nZ1
(1)
lZ1
Thermal storage water tank is usually used in an actual project of WLHPS to improve the effect of energy saving of the system. So when the quantity of heat, DQ, is added in the water-loop, the change of temperature of the system is, Dt Z
DQ rVc
(2)
The temperature inside the water-loop at a time is the difference of the temperature at the previous time plus the value of Dt. Generally the temperature in the water-loop is kept at a range of 15–35 8C. At this time cooling tower and boiler need not to work during the range of temperature. Therefore, the whole system’s performance coefficient of GHP-WLHPS, SCOP, can be expressed as: Pa P ðQe;c Þk C blZ1 ðQc;h Þl SCOP Z kZ1 P (3) aCb iZ1 ðQfuel Þ When the temperature inside the water-loop reaches below the lower limit of the temperature range, the boiler in the system will start to work to supply additional heat to the water-loop. In such a case, the SCOP can be written as: Pa Pb kZ1 ðQe;c Þk C lZ1 ðQc;h Þl SCOP Z (4) PaCb ðQ Þ C Wboil fuel iZ1 When the temperature inside the water-loop goes above the upper limit of the range, the cooling tower in the system will begin to work to take away the unwanted heat in the water-loop. In such a case, the SCOP can be got as, Pa P ðQe;c Þk C blZ1 ðQc;h Þl SCOP Z kZ1 (5) PaCb iZ1 ðQfuel Þ C Wtower
Fig. 3. Schematic view of the outdoor unit of a GHP and WLHPS.
2.2.1. Part-cooling and part-heating mode This mode may be very common-sighted in such a building with inner zone and outer zones, where some of the GHP packages work in cooling mode and others work in heating mode. Suppose that there are a sets of GHP packages running in cooling mode and b sets in heating mode in the system, the total heat quantity rejected by the cooling-mode GHP units to the water-loop can be noted as Pa kZ1 ðQc Þk , the total heat absorbed by the heating-mode P GHP units from the water-loop can be noted as blZ1 ðQe Þl , the total heat recovered from the exhaust gas of all GHP units that enters into the water-loop in the system can be PaCb ðQex Þn . Thus, the heat quantity from the GHP noted as nZ1 units to the water-loop can be expressed as:
2.2.2. All-cooling mode In this situation, all the GHP packages in the system are working in cooling mode. The heat rejected by the condenser and recovered from the exhaust gas in each GHP package will enter into the water-loop. And then it may be utilized as hot water supply or carried away by a cooling tower. The worst situation is that no hot water requirement exists and all of the heat has to be taken away by the cooling tower. At this time, the coefficient of cooling performance of the WLHPS can be expressed as: PaCb ðQ Þ SCOP Z PaCb kZ1 e;c k (6) ðQ fuel Þ C Wtower iZ1 Unfortunately, the SCOP of the new system may be lower than that of EHP-WLHPS in this case because the heat recovered from the exhaust gas of GHP units will not be utilized any longer. On the contrary, it will increase the temperature in the water-loop and lower the cooling performance of each of the GHP units in the system. Therefore, the new system can not be applied anywhere.
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2.2.3. All-heating mode In this mode, all the GHP packages in the system are working in heating mode. The heat recovered from the exhaust gas can be utilized by the GHP packages because their evaporator will absorb heat from the water-loop: PaCb ðQc;h Þk SCOP Z PkZ1 (7) aCb iZ1 ðQfuel Þ The calculations of each quantity of Qc,h, Qe, Qe,c, Qex will be directly affected by the temperature in the water-loop as well as the characteristics of the GHP units. This may be discussed later. And the detailed models of WLHPS and its analyses of energy consumption can be referred in Ref. [7]. 2.3. Characteristics of the new system The characteristics of the new system are illuminated with the help of an actual project Taking an actual project in Xi’an, a representative city in the northwest in China, as an example, the new system of the combination of GHP and WLHPS was tried to be applied. And another water/air multi split system (EHP-WLHPS) and the commonly used CACS (fan-coil system is used here) are also designed to be applied, respectively, to the same building in order to compare, analyze and evaluate their technical and economical characteristics. 2.3.1. Design of the new system applied in a building 2.3.1.1. Brief introduction to the project. The building in the project is an intelligent office building with 50 m in height and with the area of 20,000 m2 There are 11 floors above the ground, where 1–4 are department store and 5–11 are large space office rooms. Only one floor underground is available for equipment and depot. The outdoor calculation dry-bulb air temperature for air conditioning in Xi’an is K8 8C for winter and 35.2 8C for summer. The outdoor calculation wet-bulb air temperature is 26 8C for summer. The atmospheric pressure is 96.41 kPa in winter and 95.92 kPa in summer. The air conditioning environment in the building is required as, for the department store, 18 8C of temperature and more than 30% of relative humidity (RH) in winter, and 26 8C and about 60%RH in summer. The amount of fresh air is required as 8.5 m3 hK1.person both for the winter and summer. For office, 20 8C and more than 30%RH in winter, and 26 8C and about 50%RH in summer. The amount of fresh air is required as 30 m3 hK1.person both for the winter and summer. 2.3.1.2. Cooling/heating load calculation of the building. Cooling load factor method [8,9] was used for the cooling load calculation of the building with the consideration of the character of intelligent building and department store, the more heat gain from indoor heat sources. The calculation
method of the cooling load caused by outdoor affecting factors is the same as the traditional one and so is the calculation for heating load in winter, therefore, both of them are omitted here. The total cooling load of the project was about 3000 and 1500 kW for heating load. The cooling load of the standard floor is 138.9 kW, which 71.9 kW is in the outer zone and 67 kW in the inner zone, account for 51.8 and 48.2% of the total, respectively, if the depth of the outer zone is taken as 3.6 m for the calculation. 2.3.1.3. Choose and arrangement of GHP package. Watercooled heat pump packages must be used in WLHPS and no water-cooled GHP package has been developed until now. Considering the Multi-Indoor-Unit GHP has many things similar to that of WLHPS, therefore, the MIU GHP packages are supposed to be changed into water-cooled packages in order to analyze the effect of combination of GHP and WLHPS. Choose of the GHP package was based on the following principles, (a) The capacity of cooling and heating supply of GHP packages should meet the requirements of the design loads both in summer and in winter for the outer air conditioned zone at the same time because of the bigger depth of the rooms. As to the inner zone, GHP packages were chosen just based on the design cooling load in summer. (b) The cooling capacity of GHP packages was calibrated under rating condition. So it should be modified when GHP packages were applied under a condition different from that. The modification method was omitted here, which could be seen in Ref. [10] in detailed. Therefore, GHP packages were chosen based on the principles above. For example, the Multi-Indoor-Unit GHP packages with the type of LSGP were chosen to be used at the standard floor in the building, which total numbers, including inner and outer zone, was 18 for indoor units and two for outdoor units. Their specifications are shown in Table 1. Considering the building may be hired by different users, the air conditioning systems are separately set based on different floors and on the inner and outer zones in order to furthest bring the advantage of WLHPS into play and be more convenient for charging and management. Since no equipment floor in the building, the outdoor units of floor 1–4 for department stores were arranged in the basement and those of floor 5–10 for offices on the roof, which the maximum height differences between the indoor and outdoor units are 23.5 and 24 m, respectively. 2.3.1.4. The design of water loop system. Because condenser, evaporator, etc., of an EHP are usually put
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Table 1 Specifications of the multi-indoor-unit GHP
Type Number Cooling capacity (kW) Heating capacity (kW) Electric power (kW) Gas flow rate (Nm3 hK1) Air flow rate (m3 hK1) or Engine power rating (kW) Start/stop Weight (kg) Noise (dB(A)) Length of piping (m) Max height difference (m)
Indoor unit
Outdoor unit
LSGP-SH90J1 1–24 9.0 10.6 0.174 (Cooling) 0.140 (Heating) – – 1140 – Individually 22 37 120 50 (30 when position of outdoor unit lower than indoor unit)
LSGP-H560J1GU 1 56.0 67.0 1.26 (Cooling) 1.34 (Heating) 3.40(LNG)/1.56(LPG) (Cooling) 3.60(LNG)/1.65(LPG) (Heating)
together (or not far away from each other) in a package in WLHPS, the water-loop has to connect the heat-exchangers (condenser or evaporator) in each room in different floors. Considering the multi-indoor-unit GHP packages were applied in the project, so the water-loop just needed to connect all of the outdoor units, which was much more simple than that in EHP-WLHPS since all the outdoor units were arranged on the roof or in the basement. Two small water-loop systems were designed for the outdoor units on the roof and in the basement, respectively. Other equipment such as cooling tower, auxiliary heat source, etc., was still shared together, however, and was placed on the roof. Like EHP-WLHPS, the opening cooling tower combined with the fire protection water tank were adopted in the project, where electronic water treatment device was set in the system and others were similar to that in EHP-WLHPS. It can be seen in Ref. [11] for detailed. 2.3.1.5. Auxiliary heat source. Capacity of auxiliary heat source was depended on the property of the building and on the performance of the packages. Ref. [10] suggested that 2/3–1/3 of the total heating load should be taken as the capacity of auxiliary heat source in EHP-WLHPS and 70% of the value was taken in another actual project in China [12]. In the project here, the value was taken as 2/3 for safety when EHP was used. GHP has the unique advantage of stable heating capacity with the decrease of outdoor air temperature, so theoretically speaking, no auxiliary heat source is needed for GHP-WLHPS. But for safety it was still designed to be used and 1/5 of the total heating load was taken as the capacity. The total heating load of the building in winter was 1500 kW, so an electric boiler with rating power of 300 kW was chosen as an auxiliary heat source. Whether it is necessary or not to setup an auxiliary heat
15.0 Individually 930 58
source is expected to be examined in its actual running effect. 2.3.1.6. Others. Other design items for GHP-WLHPS, such as the choose of cooling tower and water pump, design of condensing water system, automation control of the system [13] and so on are almost the same in principle as that of EHP-WLHPS, except the capacities of the cooling tower and the water pumps of engine-driven units usually have to be larger than those of the electrically-driven units. The detailed design procedures are omitted here. 2.3.2. Technical comparisons The technical characteristics of EHP-WLHPS compared to CACS are also true for GHP-WLHPS, but some extra points of GHP compared to EHP are as follows, (1) GHP cascade thermal utilization system reduces primary energy consumption for heating and hot water supply. This permits higher performance and lower running cost than EHP. (2) GHP system is particularly suitable to be applied in the cases of hotels, hospitals, etc., where a lot of hot water demands exist, since much more waste heat can be recovered from GHP system. (3) The wide applications of GHP can effectively mitigate the electricity peak demand in summer and can simultaneously balance gas consumptions year after year. Its function like adjusting energy configuration will have a great social effect as well as economic effect. (4) Hydrodynamics balance in the network is very important in the design of water-loop system in EHPWLHPS. The same route system is usually adopted and balance valves need to be installed at each of main
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branches in the system in order to assure the water flow rate distributed to each of the units is what it needs. And sometimes the water-loop system needs to be divided into several parts vertically, therefore, the design of water-loop system of EHP-WLHPS is more complex and the same is to its installation. The MIU GHP doesn’t have these disadvantages, however. (5) GHP, not like EHP, has the unique advantage of stable heating capacity with the decrease of outdoor air temperature, so no (or very little capacity if necessary) auxiliary heat source is needed for GHP-WLHPS, which simplifies the design as well as saving the corresponding initial and operating cost. (6) The bigger depth in the building will make it difficult to place the compressor far away from the indoor unit of EHP package. So the noise level of EHP-WLHPS is relatively higher, in spite of taking some measures, for example, adopting the low-noise package, choosing the lower air velocity inside the ducts and using softconnection at air supply and return outlets, and so on. GHP-WLHPS using the MIU packages doesn’t have these disadvantages, however. Taking indoor unit of LSGP-SH90J1 as an example, the indoor noise level is only 37dB(A). (7) The response of GHP package is a little slower because of its large system and, system dynamics are easily changed by the installation and the ambient conditions. Therefore, it is difficult to adapt to these changes by conventional control methods. Some other new methods have to be presented [14]. 2.3.3. Economical comparison Former researches showed that initial cost of GHP package varied greatly with different situation For example, it could be 19% higher than that of EHP [15], or 16% higher when got support from government and sometimes could reach 60% higher than that of EHP when no support from government [16]. And considering the water-cooled GHP packages must be used in WLHPS, we have to reckon the initial cost of GHP-WLHPS based on other finished projects. Usually the initial cost of water-cooled package in China is 25–35% less than that of air-cooled package when water source need not be considered (water circulation system is adopted in WLHPS so it can be thought as belonging to this situation). Just like EHP-WLHPS, the initial cost of GHP-WLHPS mainly consisted of 2 parts, GHP packages and the water-loop system. Therefore, considering the situation above synthetically, the initial cost of GHP-WLHPS is usually higher than that of EHPWLHPS. And 3 situations (the same initial cost, 15% higher and 30% higher) were calculated and analyzed in the paper, and they were in the expressions of GHP-WLHPS-1, GHPWLHPS-2 and GHP-WLHPS-3, respectively, for the convenience of identification. Besides the initial cost, the operating cost also has an
important effect on the economics of the air conditioning system. And either GHP or WLHPS can save operation cost greatly, but it is true in some cases, for example, the operation cost saving of GHP changes with northern and southern cities in USA [17] and WLHPS usually has the function of heat recovery when in a large building. Therefore, the following discussions are based on this. It was obtained in Ref. [5] that GHP package could save 24.5 and 21.6% of operation cost than EHP in summer and 43.8 and 34.4% in winter, respectively, after comparing the applications of GHP and EHP package in an office building in Chicago and Dallas in the United States. It is true in Korea. Ref. [16] showed that GHP package could save 42% of operation cost than EHP for school building and 33% for business building. And even more, GHP could save as much as 58.5% of operation cost than EHP package [15]. Simultaneously, the operation cost of water-cooled package is 30–40% less than that of air-cooled one in China. Therefore, considering the situation above synthetically, the operation cost of GHP-WLHPS was taken 30–60% less than that of EHP-WLHPS. And also 3 situations of the operation cost, 30, 45 and 60% saved, respectively, were calculated and analyzed in the paper. It can be known from Ref. [11] that the initial cost was RMB8,068,000 ($975,600) when CACS was applied in the same project, and RMB8,255,000 ($998,200) for EHPWLHPS. The capacity of the electric power for CACS was 1043 kW. Two gas boilers were used for heating in winter (with the type of CWN51.16–95/70-YQ, and rating power of 900 kW). The capacity of the electric power for EHPWLHPS was 783 kW. An electric boiler with the type of CZDR-1260 was used as auxiliary heater in winter. Its rating power was 1000 kW. The building in the project was an intelligent office building in Xi’an city in China. Based on the situation of office buildings in Xi’an, the air conditioning system in this project was considered to be working from 8:00 to 18:00 in weekdays and 4 months, respectively, for winter, summer and transition seasons (spring and autumn, among them half time CACS works at all-fresh-air mode which has no energy consumption) in a whole year. No overtime work was considered at weekends and holidays (that would be more favorable for WLHPS if considered). And the auxiliary heater was thought to be working in all working hours in half a month in winter. Electricity charge for operation was RMB0.60 ($0.07) (kWh)K1 and gas price was RMB1.45 ($0.18) (Nm)K3. Simple payback method was adopted to calculate and compare economic characteristics of GHPWLHPS and EHP-WLHPS compared to CACS since there are too many factors influencing it, including not only the initial cost, but also the interest rate in a bank and escalation rate of energy. Therefore, the payback period, m, can be expressed in Eq. (8), mZ
Es2 K Es1 Ep1 K Ep2
(8)
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Table 2 Payback periods of GHP-WLHPS and EHP-WLHPS compared to CACS System
CACS EHP-WLHP GHPWLHPS-1 GHPWLHPS-2 GHPWLHPS-3
Initial cost ($103)
30% of operation cost saved
45% of operation cost saved
60% of operation cost saved
Operation cost saved ($103 yK1)
Payback period (y)
Operation cost saved ($103 yK1)
Payback period (y)
Operation cost saved ($103 yK1)
Payback period (y)
975.6 998.2 998.2
– 29.0 67.6
– 0.8 0.3
– 29.0 86.9
– 0.8 0.3
– 29.0 106.3
– 0.8 0.2
1147.9
67.6
2.5
86.9
2.0
106.3
1.6
1297.7
67.6
4.8
86.9
3.7
106.3
3.0
The results are listed in Table 2. It can be seen from Table 2 that the payback period of EHP-WLHPS is no more than one year when compared to CACS because there is not big difference of the initial cost between them. As to GHPWLHPS, its payback period is at least 3 months and 4.8 years at worst when compared to CACS under the conditions of its initial cost being in the range of 0–30% of that of EHP-WLHPS and its operation cost being saved 30–60% of that of EHP-WLHPS. Table 1 shows the comparisons between GHP-WLHPS and CACS, and between EHP-WLHPS and CACS. If comparing the applications of GHP and EHP in WLHPS, and supposing the assumptions of initial and operation cost are as the same as those above, we can obtain that, compared to EHP in this project, GHP can save $67,600 of operation cost each year at the beginning under the most favorable situation. But under the most unfavorable situation, the payback period of GHP will be 7.8 years. On the average, its payback period is of 2.6 years. What needs to say more is that the affecting factors of water-cooled and air-cooled packages were included in the comparisons above. To sum up, GHP-WLHPS has much more technical and economic advantages over EHP-WLHPS as well as over CACS in this project. It is an air conditioning system that can save more energy and worth being applied and popularized. 2.4. Several problems in the popularization of the new systems 2.4.1. Develop of water-cooled GHP package It has been said above that water-cooled heat pump packages must be used in WLHPS and no water-cooled GHP package has been developed until now. Choosing the MIU GHP package was really of no choice except this in order to analyze the effect of combination of GHP and WLHPS. It can be seen from the analysis above that the combination of GHP and WLHPS has much more technical and economic advantages over EHP-WLHPS as well as over
CACS. Therefore, it is very necessary to develop the watercooled GHP package no matter whether it is in the type of MIU GHP or of unitary package. Although the water-loop of the combination of unitary package water-cooled GHP packages and WLHPS is more complicated and its installation will take much more time, its price will be much cheaper which can reduce the initial cost of GHPWLHPS effectively. 2.4.2. Initial cost of water-cooled GHP package It has been said above that, when compared to EHPWLHPS, the payback period of GHP-WLHPS will be 7.8 years under the most unfavorable situation. And the life of GHP package is just about 10 years. So only from this point it seems that there is not very necessary to popularize it widely. But what needs to be paid attention to is that the conclusion was got under the most unfavorable situation. When on the average, its payback period is just of 2.6 years. And when compared to CACS, its payback period is just about 2 years also under the average condition. Just as the introduction above, the wide application of GHP packages can play an important social and economic role by effectively balancing electricity demand and adjusting energy configuration. Fig. 4 shows that electricity consumption of air conditioning accounts for 30% of the total in Korea. And the event of limitation to the use of electricity happened in 2/3 of provinces in China in the summer of 2003 was also resulted from the main contributions of electric air conditioners. So mitigating the electricity peak demand in summer effectively looks very important. Especially for the situation of gas transmission form west to east in China, which a mass of natural gas will be transmitted to eastern developed regions, GHP package will be a very good consumer. Therefore, one of the most effective ways to popularize GHP widely is reducing its initial cost greatly. The policies issued by government are very important besides continuous technical renovations of the GHP package itself. It can be realized by carrying out the policies such as low interest loan, tax derating, etc., to encourage the users to make full
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Fig. 4. Contribution of air conditioning to electricity consumption in Korea.
use of the clean energy. For example, the gas price is only $0.2 (Nm)K3 for cooling in Korea and the users with GHP of cooling capacity below 30RT can not only get low interest loan but also their tax can be derated as much as 10% of the initial cost of GHP. And at the same time, $2100 can be got for each package as its assistance. 2.4.3. Building division design Effect of energy saving resulting from the divisions of inner and outer zones inside a building has not been fully considered For example, if the divisions of the space inside a building is based on the depth of outer zone, say 3.6 or 4.5 m, etc., different or even opposite loads in inner and outer zones may result in different working states of heat pump packages, which can save energy furthest. Since no consideration of the effect of energy saving resulting from the divisions of inner and outer zones inside the building in the project, GHP packages were not chosen according to the inner and outer zones but to its original divisions. So all the offices that had outside wall were taken as a system, here called outer zone system. Calculation results showed that the cooling load of the outer zone system accounted for 68% of that in the standard floor. All the offices that didn’t have outside wall were taken as another system, here called inner zone system, which cooling load accounted for 32% of that in the standard floor. The two percentage values were 51.8 and 48.2%, respectively, if the depth of the outer zone was taken as 3.6 m for the calculation.
The difference of number of indoor unit caused by strict division of inner and outer zones can be illuminated by the example of choosing the indoor GHP units in the standard floor (Table 3). Table 3 shows that the total number of the indoor units in each floor is of 22 of the type of LSGP when the inner and outer zones are not strictly divided. And even if the rooms which are near each other in the inner zone can share one unit, the number is still of 18. But it will just be 14 of units in each floor when they are strictly divided. More units are need is only one of the shortage resulted from no-strict-division of inner and outer zones. The more important is that it is easier to cause the indoor temperature field un-uniform if the requirement of cooling and heating is existed simultaneously in a big room.
3. Conclusions (1) Conception of combination of GHP and WLHPS is presented in the paper in order to reduce the energy consumption of air conditioning system further. And the design of the new system applied in a building is introduced through an actual project in China. (2) Comparisons of the characteristics between GHPWLHPS and CACS, and between GHP-WLHPS and EHP-WLHPS show that, as far as this project is concerned, the technology of GHP-WLHPS is reasonable and its economic characteristics are better. So it is worth being applied and popularized.
Table 3 Difference of number of indoor unit caused by strict division of inner and outer zone Item
No-strict-division of inner and outer zone
Strict-division of inner and outer zone
Inner zone area (m2) Cooling load in inner zone (kW) Outer zone area (m2) Cooling load in outer zone (kW) Type of package in inner zone Type of package in outer zone Total number of packages in standard floor
350 23.7 1280 115.2 7 of SH36J1 (3 of SH90J1) 15 of SH90J1 22(18)
990 67.0 640 71.9 7 of SH112J1 7 of SH112J1 14
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(3) It is very necessary to develop the water-cooled GHP package no matter whether it is in the type of MIU GHP or of unitary package. We have to admit that the higher initial cost of GHP package is its main obstacle of the application and popularization. (4) It is suggested that the effect of energy saving resulting from the divisions of inner and outer zones inside a building should be fully considered in order to save energy furthest. (5) Considering the social and economic roles of balancing electricity demand and adjusting energy configuration of GHP, the corresponding policies should be issued by government in order to encourage the users to make full use of the clean energy.
Acknowledgements Thank the Brain Pool Program sponsored by the KOSEF (Korea Science and Engineering Foundation) and KOFST (Korean Federation of Science and Technology Societies) for their financial support for the author’s working in Korea. And the authors also want to express thanks to Ms A.L. Lian for her translation of the paper.
References [1] D.D. Colosimo, Introduction to engine-driven heat pumpsconcept, approach, and economics, ASHRAE Trans 93 (1987) 987–996. [2] F. Cascetta, M. Sasso, S. Sibilio, A metrological analysis of in situ evaluation of the performance of a gas engine-driven heat pump, Measurement 16 (1995) 209–217. [3] T. Nakamura, Trends of gas engine-driven heat pump systems in Japan. Proceeding of the sixth international energy agency heat pump conference, Berlin, Germany; 1999: Poster Presentation 1. [4] L.A. Howe, R. Radermacher, K.E. Herold, Combined cycles for engine-driven heat pumps, Int J Refrigeration 12 (1) (1989) 21–28.
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[5] R.E. Monahan, K.J. Kountz, J.M. Clinch, The development of a kinematic Stirling-engine-driven heat pump, ASHRAE Trans 93 (1987) 1021–1032. [6] X. Li, Thermal performance and energy saving effect of waterloop heat pump systems with geothermal [J], Energ Convers Manage 39 (3) (1998) 295–346. [7] Z. Lian, S.-R Park, H. Qi, Analysis on energy consumption of water-loop heat pump system in China, J Appl Therm Eng 25 (1) (2005) 73–85. [8] ASHRAE, Handbook of fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, USA, 1997. [9] Z. Lian, Y. Zhang, Distribution ratio of radiant heat and its effect on cooling load [J], Int J Therm Sci 42 (3) (2003) 311– 316. [10] S.W. Lang, W. Xu, T.S. Feng, Issues of design and application of the centralized air conditioning system with water source heat pump units, J HVAC 26 (1) (1996) 15–19 [in Chinese]. [11] Z.W. Lian, S.R. Park, W. Huang, Comparisons and analysis of application between water-loop heat pump system and traditional air conditioning system in China, Proceeding of KOCS international symposium, Seoul, Korea, vol. 4–6, 2003. p. 34–43. [12] R.F. Ye, S.C. Wang, Application of the centralized air conditioning with water-loop heat pump system in a project, J HVAC 27 (6) (1997) 50–54 [in Chinese]. [13] M. Zaheer-Uddin, J.C.Y. Wang, Start–stop control strategies for heat recovery in multi-zone water heat pump systems, J Heat Recovery Syst CHP 12 (4) (1992) 335–346. [14] Y. Hasegawa, T. Nakano, T. Fukuda, et al. Learning predictive control for GHP. Industrial Electronics Society; 1999. IECON’99 Proceedings. The 25th annual conference of the IEEE, 1999, v.3: p. 1277–1282. [15] D.H. Lee, GHP system installation examples in Korea. Korean association of air conditioning, Refrigerating Sanitary Eng 19 (4) (2002) 64–68 [in Korean]. [16] J.O. Han, Economical efficiency evaluation of GHP system. Korean association of air conditioning, Refrigerating Sanitary Eng 19 (4) (2002) 73–81 [in Korean]. [17] R.D. Fischer, F.E. Jakob, C.E. French, Engine-driven heat pump performance analysis, ASHRAE Trans 93 (1987) 1046– 1077.
Conception of combination of gas-engine-driven heat pump and water-loop heat pump system Zhiwei Liana,*, Seong-ryong Parkb, Wei Huangc, Young-jin Baikb, Ye Yaoa a b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China Department of Energy Efficiency, Korea Institute of Energy Research, Taejeon 305-343, South Korea c Shanxi Building Design and Research Institute, Shanxi 710001, China Received 6 October 2003; received in revised form 31 December 2004; accepted 28 February 2005 Available online 26 May 2005
Abstract Conception of combination of gas-engine-driven heat pump (GHP) and water-loop heat pump system (WLHPS) is firstly presented in the paper in order to reduce the energy consumption of air conditioning system further. Design of the new system is introduced through an actual project in China and compared with a conventional air-conditioning system (CACS) and conventional WLHPS (EHP-WLHPS) in terms of technical characteristics and payback period. It is found that the payback period of GHP-WLHPS is about 2 years when compared with CACS and 2.6 years with EHP-WLHPS on the average. So it is worth being more widely applied. And several barrier to application are also discussed. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Air conditioning; China; Heat pump; Gas engine; Modelling; Hydraulic circuit; Comparison; Payback
Conception d’une pompe a` chaleur entraıˆne´e par un moteur a` gaz relie´e a` un syste`me de pompe a` chaleur a` boucle d’eau Mots cle´s: Conditionnement d’air ; Chine ; Pompe a` chaleur ; Moteur a` gaz ; Mode´lisation ; Circuit hydraulique ; Comparaison ; Amortissement e´conomique
1. Introduction It is well known that more and more energy is being consumed in air conditioning system in almost every country in the world. And much attention has been paid on it. Heat pump is known as a kind of energy saving air conditioning
* Corresponding author. Tel.: C86 21 62933240; fax: C86 21 62932601. E-mail address: [email protected] (Z. Lian).
0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.02.004
equipment, which is mostly driven by electricity. Current practice is mainly to convert fuel to electric power at a central power plant and reject the waste heat to the environment. The electricity is then transmitted to the heat pump motor, which provides the work input to the heat pump cycle. As is seen from Fig. 1, significant losses will occur from fuels to work of the heat pumps in such a case [1]. An obvious way to improve overall fuel utilization is to locate the fuel conversion process closer to, where the exhaust heat can be productively used. A combustion engine directly driving a heat pump compressor has about the same
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Nomenclature a b c CACS COP COPh EHP Ep1 Ep2 Es1 Es2 GHP m Qc Qc,h
number of GHP packages running in cooling mode number of GHP packages running in heating mode specific heat of water in the loop, kJ/kg 8C conventional air conditioning system co-efficient of cooling performance of GHP co-efficient of heating performance of GHP electric-driven heat pump first year or annual energy cost of CACS, $ first year or annual energy cost of GHPWLHPS or EHP-WLHPS, $ initial cost of CACS, $ initial cost of GHP-WLHPS or EHP-WLHPS, $ gas-engine-driven heat pump payback period, year heat quantity rejected to the water-loop by GHP unit in cooling mode, kJ heat quantity provided by GHP unit in heating mode, kJ
overall fuel-to-heat or cooling efficiency as an electric unit. An additional advantage is that the combustion engine’s waste heat can be utilized at the site to provide supplemental space heating or water heating. Therefore, gas-enginedriven heat pump (GHP) will have a high efficiency of fuel utilization. At this time no more than 1/3 of energy consumed in a conventional heat pump is needed in GHP to achieve the same heating effect (Fig. 2) [1]. This improved utilization of fuel is one of the most important factors that have given rise to the development of unitary GHP package in Japan, Europe, and the USA [2–5]. And the development of the GHP package in Japan was also the result of regulation for reducing the peak load in the electric grid. Just like the air-cooled heat pumps, all the GHP packages now are of air-cooled type and some energy is still wasted when heat is transferred between external air and the heat exchanger inside the outdoor unit, especially when there is different requirements of cold and heat inside a building. Water-loop heat pump system (WLHPS), which
Qe
heat absorbed by GHP unit in heating mode from the water-loop, kJ Qe,c cooling quantity provided by GHP unit in cooling mode, kJ Qex heat quantity recovered from the exhaust gas of GHP, kJ Qfuel heat quantity produced by fuel supplied to GHP, kJ SCOP whole system’s performance coefficient of GHP-WLHPS Wboil energy consumed by boiler in GHP-WLHPS, kJ Wtower energy consumed by cooling tower in GHPWLHPS, kJ WLHPS water-loop heat pump system V volume of water in WLHPS, m3 r density of water in the loop, kg/m3 DQ heat quantity transferred from GHP units to water-loop, kJ Dt change of temperature of the water in WLHPS, 8C
has the function of heat recovery by transferring the heat in the inner zone (core area) to the outer zone (perimeter area) by the circulation water, makes it possible to use this part of energy [6]. Therefore, this paper presents a new conception of combination of GHP and WLHPS in order to save energy further in air conditioning systems.
2. Conception of combination of GHP and WLHPS Although GHP was appeared on the Japanese HVAC marketplace in 1986 and WLHPS even as early as 1960s in the USA, the combination of GHP and WLHPS is completely a new conception of their application. So some discussions on the new system are presented in the paper. 2.1. Description of the new system As to a GHP package, its typical system is split into
Fig. 1. Losses of conversion process from fuels to work of a heat pump.
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Fig. 2. Comparison of energy conversion process between conventional EHP and GHP.
2 parts: one outdoor unit and one (or several) indoor one(s). A schematic view of the outdoor unit of a water-cooled GHP package, operating in heating mode, is shown in Fig. 3(a). In the case of heating mode, the refrigerant-water heat exchanger in Fig. 3(a) works as an ‘evaporator’ that absorbs heat from the water in the water-loop. While in the case of cooling mode, it works as a ‘condenser’ that rejects heat to the water in the water-loop. It can be seen that waste heat is recovered both from the engine jacket and from the exhaust by means of a compact heat exchanger. Those parts of heat can be transferred to heat the water in the water-loop in order to improve the performance of WLHPS in winter or to be used for hot water supply in a whole year. And besides, the heating performance of the GHP system can be enhanced by raising the lower pressure of the refrigerant in winter and the efficiency of the gas engine in the GHP package will also be improved because the heat quantity of the exhaust gas decreases after the heat recovery.
Fig. 3(b) shows the schematic view of WLHPS, where the GHP packages can be applied, among them connecting points are A and B, respectively, in Fig. 3(a) and (b). Much more energy can be saved especially when they are applied to such a building with inner zone and outer zones, where some of the GHP packages work in cooling mode while some work in heating mode. At this time, the heat rejected from the GHP packages work in refrigeration mode inside the building could be transferred to the outer zone by the circulation water in the water-loop and heat recovery could be realized. 2.2. Performance model of the new system To explain the performance of the GHP-WLHPS, its performance model is discussed in three cases as ‘all cooling’, ‘all heating’, and ‘part cooling and part heating’, respectively.
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DQ Z
813
a aCb b X X X ðQc Þk C ðQex Þn K ðQe Þl kZ1
nZ1
(1)
lZ1
Thermal storage water tank is usually used in an actual project of WLHPS to improve the effect of energy saving of the system. So when the quantity of heat, DQ, is added in the water-loop, the change of temperature of the system is, Dt Z
DQ rVc
(2)
The temperature inside the water-loop at a time is the difference of the temperature at the previous time plus the value of Dt. Generally the temperature in the water-loop is kept at a range of 15–35 8C. At this time cooling tower and boiler need not to work during the range of temperature. Therefore, the whole system’s performance coefficient of GHP-WLHPS, SCOP, can be expressed as: Pa P ðQe;c Þk C blZ1 ðQc;h Þl SCOP Z kZ1 P (3) aCb iZ1 ðQfuel Þ When the temperature inside the water-loop reaches below the lower limit of the temperature range, the boiler in the system will start to work to supply additional heat to the water-loop. In such a case, the SCOP can be written as: Pa Pb kZ1 ðQe;c Þk C lZ1 ðQc;h Þl SCOP Z (4) PaCb ðQ Þ C Wboil fuel iZ1 When the temperature inside the water-loop goes above the upper limit of the range, the cooling tower in the system will begin to work to take away the unwanted heat in the water-loop. In such a case, the SCOP can be got as, Pa P ðQe;c Þk C blZ1 ðQc;h Þl SCOP Z kZ1 (5) PaCb iZ1 ðQfuel Þ C Wtower
Fig. 3. Schematic view of the outdoor unit of a GHP and WLHPS.
2.2.1. Part-cooling and part-heating mode This mode may be very common-sighted in such a building with inner zone and outer zones, where some of the GHP packages work in cooling mode and others work in heating mode. Suppose that there are a sets of GHP packages running in cooling mode and b sets in heating mode in the system, the total heat quantity rejected by the cooling-mode GHP units to the water-loop can be noted as Pa kZ1 ðQc Þk , the total heat absorbed by the heating-mode P GHP units from the water-loop can be noted as blZ1 ðQe Þl , the total heat recovered from the exhaust gas of all GHP units that enters into the water-loop in the system can be PaCb ðQex Þn . Thus, the heat quantity from the GHP noted as nZ1 units to the water-loop can be expressed as:
2.2.2. All-cooling mode In this situation, all the GHP packages in the system are working in cooling mode. The heat rejected by the condenser and recovered from the exhaust gas in each GHP package will enter into the water-loop. And then it may be utilized as hot water supply or carried away by a cooling tower. The worst situation is that no hot water requirement exists and all of the heat has to be taken away by the cooling tower. At this time, the coefficient of cooling performance of the WLHPS can be expressed as: PaCb ðQ Þ SCOP Z PaCb kZ1 e;c k (6) ðQ fuel Þ C Wtower iZ1 Unfortunately, the SCOP of the new system may be lower than that of EHP-WLHPS in this case because the heat recovered from the exhaust gas of GHP units will not be utilized any longer. On the contrary, it will increase the temperature in the water-loop and lower the cooling performance of each of the GHP units in the system. Therefore, the new system can not be applied anywhere.
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2.2.3. All-heating mode In this mode, all the GHP packages in the system are working in heating mode. The heat recovered from the exhaust gas can be utilized by the GHP packages because their evaporator will absorb heat from the water-loop: PaCb ðQc;h Þk SCOP Z PkZ1 (7) aCb iZ1 ðQfuel Þ The calculations of each quantity of Qc,h, Qe, Qe,c, Qex will be directly affected by the temperature in the water-loop as well as the characteristics of the GHP units. This may be discussed later. And the detailed models of WLHPS and its analyses of energy consumption can be referred in Ref. [7]. 2.3. Characteristics of the new system The characteristics of the new system are illuminated with the help of an actual project Taking an actual project in Xi’an, a representative city in the northwest in China, as an example, the new system of the combination of GHP and WLHPS was tried to be applied. And another water/air multi split system (EHP-WLHPS) and the commonly used CACS (fan-coil system is used here) are also designed to be applied, respectively, to the same building in order to compare, analyze and evaluate their technical and economical characteristics. 2.3.1. Design of the new system applied in a building 2.3.1.1. Brief introduction to the project. The building in the project is an intelligent office building with 50 m in height and with the area of 20,000 m2 There are 11 floors above the ground, where 1–4 are department store and 5–11 are large space office rooms. Only one floor underground is available for equipment and depot. The outdoor calculation dry-bulb air temperature for air conditioning in Xi’an is K8 8C for winter and 35.2 8C for summer. The outdoor calculation wet-bulb air temperature is 26 8C for summer. The atmospheric pressure is 96.41 kPa in winter and 95.92 kPa in summer. The air conditioning environment in the building is required as, for the department store, 18 8C of temperature and more than 30% of relative humidity (RH) in winter, and 26 8C and about 60%RH in summer. The amount of fresh air is required as 8.5 m3 hK1.person both for the winter and summer. For office, 20 8C and more than 30%RH in winter, and 26 8C and about 50%RH in summer. The amount of fresh air is required as 30 m3 hK1.person both for the winter and summer. 2.3.1.2. Cooling/heating load calculation of the building. Cooling load factor method [8,9] was used for the cooling load calculation of the building with the consideration of the character of intelligent building and department store, the more heat gain from indoor heat sources. The calculation
method of the cooling load caused by outdoor affecting factors is the same as the traditional one and so is the calculation for heating load in winter, therefore, both of them are omitted here. The total cooling load of the project was about 3000 and 1500 kW for heating load. The cooling load of the standard floor is 138.9 kW, which 71.9 kW is in the outer zone and 67 kW in the inner zone, account for 51.8 and 48.2% of the total, respectively, if the depth of the outer zone is taken as 3.6 m for the calculation. 2.3.1.3. Choose and arrangement of GHP package. Watercooled heat pump packages must be used in WLHPS and no water-cooled GHP package has been developed until now. Considering the Multi-Indoor-Unit GHP has many things similar to that of WLHPS, therefore, the MIU GHP packages are supposed to be changed into water-cooled packages in order to analyze the effect of combination of GHP and WLHPS. Choose of the GHP package was based on the following principles, (a) The capacity of cooling and heating supply of GHP packages should meet the requirements of the design loads both in summer and in winter for the outer air conditioned zone at the same time because of the bigger depth of the rooms. As to the inner zone, GHP packages were chosen just based on the design cooling load in summer. (b) The cooling capacity of GHP packages was calibrated under rating condition. So it should be modified when GHP packages were applied under a condition different from that. The modification method was omitted here, which could be seen in Ref. [10] in detailed. Therefore, GHP packages were chosen based on the principles above. For example, the Multi-Indoor-Unit GHP packages with the type of LSGP were chosen to be used at the standard floor in the building, which total numbers, including inner and outer zone, was 18 for indoor units and two for outdoor units. Their specifications are shown in Table 1. Considering the building may be hired by different users, the air conditioning systems are separately set based on different floors and on the inner and outer zones in order to furthest bring the advantage of WLHPS into play and be more convenient for charging and management. Since no equipment floor in the building, the outdoor units of floor 1–4 for department stores were arranged in the basement and those of floor 5–10 for offices on the roof, which the maximum height differences between the indoor and outdoor units are 23.5 and 24 m, respectively. 2.3.1.4. The design of water loop system. Because condenser, evaporator, etc., of an EHP are usually put
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Table 1 Specifications of the multi-indoor-unit GHP
Type Number Cooling capacity (kW) Heating capacity (kW) Electric power (kW) Gas flow rate (Nm3 hK1) Air flow rate (m3 hK1) or Engine power rating (kW) Start/stop Weight (kg) Noise (dB(A)) Length of piping (m) Max height difference (m)
Indoor unit
Outdoor unit
LSGP-SH90J1 1–24 9.0 10.6 0.174 (Cooling) 0.140 (Heating) – – 1140 – Individually 22 37 120 50 (30 when position of outdoor unit lower than indoor unit)
LSGP-H560J1GU 1 56.0 67.0 1.26 (Cooling) 1.34 (Heating) 3.40(LNG)/1.56(LPG) (Cooling) 3.60(LNG)/1.65(LPG) (Heating)
together (or not far away from each other) in a package in WLHPS, the water-loop has to connect the heat-exchangers (condenser or evaporator) in each room in different floors. Considering the multi-indoor-unit GHP packages were applied in the project, so the water-loop just needed to connect all of the outdoor units, which was much more simple than that in EHP-WLHPS since all the outdoor units were arranged on the roof or in the basement. Two small water-loop systems were designed for the outdoor units on the roof and in the basement, respectively. Other equipment such as cooling tower, auxiliary heat source, etc., was still shared together, however, and was placed on the roof. Like EHP-WLHPS, the opening cooling tower combined with the fire protection water tank were adopted in the project, where electronic water treatment device was set in the system and others were similar to that in EHP-WLHPS. It can be seen in Ref. [11] for detailed. 2.3.1.5. Auxiliary heat source. Capacity of auxiliary heat source was depended on the property of the building and on the performance of the packages. Ref. [10] suggested that 2/3–1/3 of the total heating load should be taken as the capacity of auxiliary heat source in EHP-WLHPS and 70% of the value was taken in another actual project in China [12]. In the project here, the value was taken as 2/3 for safety when EHP was used. GHP has the unique advantage of stable heating capacity with the decrease of outdoor air temperature, so theoretically speaking, no auxiliary heat source is needed for GHP-WLHPS. But for safety it was still designed to be used and 1/5 of the total heating load was taken as the capacity. The total heating load of the building in winter was 1500 kW, so an electric boiler with rating power of 300 kW was chosen as an auxiliary heat source. Whether it is necessary or not to setup an auxiliary heat
15.0 Individually 930 58
source is expected to be examined in its actual running effect. 2.3.1.6. Others. Other design items for GHP-WLHPS, such as the choose of cooling tower and water pump, design of condensing water system, automation control of the system [13] and so on are almost the same in principle as that of EHP-WLHPS, except the capacities of the cooling tower and the water pumps of engine-driven units usually have to be larger than those of the electrically-driven units. The detailed design procedures are omitted here. 2.3.2. Technical comparisons The technical characteristics of EHP-WLHPS compared to CACS are also true for GHP-WLHPS, but some extra points of GHP compared to EHP are as follows, (1) GHP cascade thermal utilization system reduces primary energy consumption for heating and hot water supply. This permits higher performance and lower running cost than EHP. (2) GHP system is particularly suitable to be applied in the cases of hotels, hospitals, etc., where a lot of hot water demands exist, since much more waste heat can be recovered from GHP system. (3) The wide applications of GHP can effectively mitigate the electricity peak demand in summer and can simultaneously balance gas consumptions year after year. Its function like adjusting energy configuration will have a great social effect as well as economic effect. (4) Hydrodynamics balance in the network is very important in the design of water-loop system in EHPWLHPS. The same route system is usually adopted and balance valves need to be installed at each of main
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branches in the system in order to assure the water flow rate distributed to each of the units is what it needs. And sometimes the water-loop system needs to be divided into several parts vertically, therefore, the design of water-loop system of EHP-WLHPS is more complex and the same is to its installation. The MIU GHP doesn’t have these disadvantages, however. (5) GHP, not like EHP, has the unique advantage of stable heating capacity with the decrease of outdoor air temperature, so no (or very little capacity if necessary) auxiliary heat source is needed for GHP-WLHPS, which simplifies the design as well as saving the corresponding initial and operating cost. (6) The bigger depth in the building will make it difficult to place the compressor far away from the indoor unit of EHP package. So the noise level of EHP-WLHPS is relatively higher, in spite of taking some measures, for example, adopting the low-noise package, choosing the lower air velocity inside the ducts and using softconnection at air supply and return outlets, and so on. GHP-WLHPS using the MIU packages doesn’t have these disadvantages, however. Taking indoor unit of LSGP-SH90J1 as an example, the indoor noise level is only 37dB(A). (7) The response of GHP package is a little slower because of its large system and, system dynamics are easily changed by the installation and the ambient conditions. Therefore, it is difficult to adapt to these changes by conventional control methods. Some other new methods have to be presented [14]. 2.3.3. Economical comparison Former researches showed that initial cost of GHP package varied greatly with different situation For example, it could be 19% higher than that of EHP [15], or 16% higher when got support from government and sometimes could reach 60% higher than that of EHP when no support from government [16]. And considering the water-cooled GHP packages must be used in WLHPS, we have to reckon the initial cost of GHP-WLHPS based on other finished projects. Usually the initial cost of water-cooled package in China is 25–35% less than that of air-cooled package when water source need not be considered (water circulation system is adopted in WLHPS so it can be thought as belonging to this situation). Just like EHP-WLHPS, the initial cost of GHP-WLHPS mainly consisted of 2 parts, GHP packages and the water-loop system. Therefore, considering the situation above synthetically, the initial cost of GHP-WLHPS is usually higher than that of EHPWLHPS. And 3 situations (the same initial cost, 15% higher and 30% higher) were calculated and analyzed in the paper, and they were in the expressions of GHP-WLHPS-1, GHPWLHPS-2 and GHP-WLHPS-3, respectively, for the convenience of identification. Besides the initial cost, the operating cost also has an
important effect on the economics of the air conditioning system. And either GHP or WLHPS can save operation cost greatly, but it is true in some cases, for example, the operation cost saving of GHP changes with northern and southern cities in USA [17] and WLHPS usually has the function of heat recovery when in a large building. Therefore, the following discussions are based on this. It was obtained in Ref. [5] that GHP package could save 24.5 and 21.6% of operation cost than EHP in summer and 43.8 and 34.4% in winter, respectively, after comparing the applications of GHP and EHP package in an office building in Chicago and Dallas in the United States. It is true in Korea. Ref. [16] showed that GHP package could save 42% of operation cost than EHP for school building and 33% for business building. And even more, GHP could save as much as 58.5% of operation cost than EHP package [15]. Simultaneously, the operation cost of water-cooled package is 30–40% less than that of air-cooled one in China. Therefore, considering the situation above synthetically, the operation cost of GHP-WLHPS was taken 30–60% less than that of EHP-WLHPS. And also 3 situations of the operation cost, 30, 45 and 60% saved, respectively, were calculated and analyzed in the paper. It can be known from Ref. [11] that the initial cost was RMB8,068,000 ($975,600) when CACS was applied in the same project, and RMB8,255,000 ($998,200) for EHPWLHPS. The capacity of the electric power for CACS was 1043 kW. Two gas boilers were used for heating in winter (with the type of CWN51.16–95/70-YQ, and rating power of 900 kW). The capacity of the electric power for EHPWLHPS was 783 kW. An electric boiler with the type of CZDR-1260 was used as auxiliary heater in winter. Its rating power was 1000 kW. The building in the project was an intelligent office building in Xi’an city in China. Based on the situation of office buildings in Xi’an, the air conditioning system in this project was considered to be working from 8:00 to 18:00 in weekdays and 4 months, respectively, for winter, summer and transition seasons (spring and autumn, among them half time CACS works at all-fresh-air mode which has no energy consumption) in a whole year. No overtime work was considered at weekends and holidays (that would be more favorable for WLHPS if considered). And the auxiliary heater was thought to be working in all working hours in half a month in winter. Electricity charge for operation was RMB0.60 ($0.07) (kWh)K1 and gas price was RMB1.45 ($0.18) (Nm)K3. Simple payback method was adopted to calculate and compare economic characteristics of GHPWLHPS and EHP-WLHPS compared to CACS since there are too many factors influencing it, including not only the initial cost, but also the interest rate in a bank and escalation rate of energy. Therefore, the payback period, m, can be expressed in Eq. (8), mZ
Es2 K Es1 Ep1 K Ep2
(8)
Z. Lian et al. / International Journal of Refrigeration 28 (2005) 810–819
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Table 2 Payback periods of GHP-WLHPS and EHP-WLHPS compared to CACS System
CACS EHP-WLHP GHPWLHPS-1 GHPWLHPS-2 GHPWLHPS-3
Initial cost ($103)
30% of operation cost saved
45% of operation cost saved
60% of operation cost saved
Operation cost saved ($103 yK1)
Payback period (y)
Operation cost saved ($103 yK1)
Payback period (y)
Operation cost saved ($103 yK1)
Payback period (y)
975.6 998.2 998.2
– 29.0 67.6
– 0.8 0.3
– 29.0 86.9
– 0.8 0.3
– 29.0 106.3
– 0.8 0.2
1147.9
67.6
2.5
86.9
2.0
106.3
1.6
1297.7
67.6
4.8
86.9
3.7
106.3
3.0
The results are listed in Table 2. It can be seen from Table 2 that the payback period of EHP-WLHPS is no more than one year when compared to CACS because there is not big difference of the initial cost between them. As to GHPWLHPS, its payback period is at least 3 months and 4.8 years at worst when compared to CACS under the conditions of its initial cost being in the range of 0–30% of that of EHP-WLHPS and its operation cost being saved 30–60% of that of EHP-WLHPS. Table 1 shows the comparisons between GHP-WLHPS and CACS, and between EHP-WLHPS and CACS. If comparing the applications of GHP and EHP in WLHPS, and supposing the assumptions of initial and operation cost are as the same as those above, we can obtain that, compared to EHP in this project, GHP can save $67,600 of operation cost each year at the beginning under the most favorable situation. But under the most unfavorable situation, the payback period of GHP will be 7.8 years. On the average, its payback period is of 2.6 years. What needs to say more is that the affecting factors of water-cooled and air-cooled packages were included in the comparisons above. To sum up, GHP-WLHPS has much more technical and economic advantages over EHP-WLHPS as well as over CACS in this project. It is an air conditioning system that can save more energy and worth being applied and popularized. 2.4. Several problems in the popularization of the new systems 2.4.1. Develop of water-cooled GHP package It has been said above that water-cooled heat pump packages must be used in WLHPS and no water-cooled GHP package has been developed until now. Choosing the MIU GHP package was really of no choice except this in order to analyze the effect of combination of GHP and WLHPS. It can be seen from the analysis above that the combination of GHP and WLHPS has much more technical and economic advantages over EHP-WLHPS as well as over
CACS. Therefore, it is very necessary to develop the watercooled GHP package no matter whether it is in the type of MIU GHP or of unitary package. Although the water-loop of the combination of unitary package water-cooled GHP packages and WLHPS is more complicated and its installation will take much more time, its price will be much cheaper which can reduce the initial cost of GHPWLHPS effectively. 2.4.2. Initial cost of water-cooled GHP package It has been said above that, when compared to EHPWLHPS, the payback period of GHP-WLHPS will be 7.8 years under the most unfavorable situation. And the life of GHP package is just about 10 years. So only from this point it seems that there is not very necessary to popularize it widely. But what needs to be paid attention to is that the conclusion was got under the most unfavorable situation. When on the average, its payback period is just of 2.6 years. And when compared to CACS, its payback period is just about 2 years also under the average condition. Just as the introduction above, the wide application of GHP packages can play an important social and economic role by effectively balancing electricity demand and adjusting energy configuration. Fig. 4 shows that electricity consumption of air conditioning accounts for 30% of the total in Korea. And the event of limitation to the use of electricity happened in 2/3 of provinces in China in the summer of 2003 was also resulted from the main contributions of electric air conditioners. So mitigating the electricity peak demand in summer effectively looks very important. Especially for the situation of gas transmission form west to east in China, which a mass of natural gas will be transmitted to eastern developed regions, GHP package will be a very good consumer. Therefore, one of the most effective ways to popularize GHP widely is reducing its initial cost greatly. The policies issued by government are very important besides continuous technical renovations of the GHP package itself. It can be realized by carrying out the policies such as low interest loan, tax derating, etc., to encourage the users to make full
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Fig. 4. Contribution of air conditioning to electricity consumption in Korea.
use of the clean energy. For example, the gas price is only $0.2 (Nm)K3 for cooling in Korea and the users with GHP of cooling capacity below 30RT can not only get low interest loan but also their tax can be derated as much as 10% of the initial cost of GHP. And at the same time, $2100 can be got for each package as its assistance. 2.4.3. Building division design Effect of energy saving resulting from the divisions of inner and outer zones inside a building has not been fully considered For example, if the divisions of the space inside a building is based on the depth of outer zone, say 3.6 or 4.5 m, etc., different or even opposite loads in inner and outer zones may result in different working states of heat pump packages, which can save energy furthest. Since no consideration of the effect of energy saving resulting from the divisions of inner and outer zones inside the building in the project, GHP packages were not chosen according to the inner and outer zones but to its original divisions. So all the offices that had outside wall were taken as a system, here called outer zone system. Calculation results showed that the cooling load of the outer zone system accounted for 68% of that in the standard floor. All the offices that didn’t have outside wall were taken as another system, here called inner zone system, which cooling load accounted for 32% of that in the standard floor. The two percentage values were 51.8 and 48.2%, respectively, if the depth of the outer zone was taken as 3.6 m for the calculation.
The difference of number of indoor unit caused by strict division of inner and outer zones can be illuminated by the example of choosing the indoor GHP units in the standard floor (Table 3). Table 3 shows that the total number of the indoor units in each floor is of 22 of the type of LSGP when the inner and outer zones are not strictly divided. And even if the rooms which are near each other in the inner zone can share one unit, the number is still of 18. But it will just be 14 of units in each floor when they are strictly divided. More units are need is only one of the shortage resulted from no-strict-division of inner and outer zones. The more important is that it is easier to cause the indoor temperature field un-uniform if the requirement of cooling and heating is existed simultaneously in a big room.
3. Conclusions (1) Conception of combination of GHP and WLHPS is presented in the paper in order to reduce the energy consumption of air conditioning system further. And the design of the new system applied in a building is introduced through an actual project in China. (2) Comparisons of the characteristics between GHPWLHPS and CACS, and between GHP-WLHPS and EHP-WLHPS show that, as far as this project is concerned, the technology of GHP-WLHPS is reasonable and its economic characteristics are better. So it is worth being applied and popularized.
Table 3 Difference of number of indoor unit caused by strict division of inner and outer zone Item
No-strict-division of inner and outer zone
Strict-division of inner and outer zone
Inner zone area (m2) Cooling load in inner zone (kW) Outer zone area (m2) Cooling load in outer zone (kW) Type of package in inner zone Type of package in outer zone Total number of packages in standard floor
350 23.7 1280 115.2 7 of SH36J1 (3 of SH90J1) 15 of SH90J1 22(18)
990 67.0 640 71.9 7 of SH112J1 7 of SH112J1 14
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(3) It is very necessary to develop the water-cooled GHP package no matter whether it is in the type of MIU GHP or of unitary package. We have to admit that the higher initial cost of GHP package is its main obstacle of the application and popularization. (4) It is suggested that the effect of energy saving resulting from the divisions of inner and outer zones inside a building should be fully considered in order to save energy furthest. (5) Considering the social and economic roles of balancing electricity demand and adjusting energy configuration of GHP, the corresponding policies should be issued by government in order to encourage the users to make full use of the clean energy.
Acknowledgements Thank the Brain Pool Program sponsored by the KOSEF (Korea Science and Engineering Foundation) and KOFST (Korean Federation of Science and Technology Societies) for their financial support for the author’s working in Korea. And the authors also want to express thanks to Ms A.L. Lian for her translation of the paper.
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