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Experimental investigation of adsorption chiller for Micro-scale BCHP system application
Energy and Buildings 39 (2007) 120–127 www.elsevier.com/locate/enbuild
Experimental investigation of adsorption chiller for Micro-scale BCHP system application Y. Huangfu, J.Y. Wu *, R.Z. Wang, Z.Z. Xia Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China Received 5 March 2006; received in revised form 3 April 2006; accepted 4 April 2006
Abstract Building cooling, heating and power (BCHP), is an attracting cogeneration system for its economic and environmental friendly qualities. Therefore, it is meaningful to develop Micro-scale BCHP (MBCHP) system based on adsorption chiller and internal combustion gas engine for the use of residential and light commercial buildings. To guide the optimum matching and operation of adsorption chiller in MBCHP system correctly, this paper deals with the performance of adsorption chiller under varying heating conditions. Experimental results show that the value of COP is high in the operation mode of varying hot water inlet temperature with mass recovery in no heating pattern (VTNH). With the hot water inlet temperature of 65 8C, the value of COP is as high as 0.40 in VTNH mode. Under electricity output conditions from 6.0 to 8.3 kW in MBCHP system, VTNH mode is especially preferred when cooling demand is with priority. # 2006 Elsevier B.V. All rights reserved. Keywords: Adsorption chiller; MBCHP; Varying heating conditions; Experimental investigation; Application
1. Introduction As a kind of distributed energy system (DES), building cooling, heating and power (BCHP) is attracting more and more concerns [1–4]. The advantage of BCHP system for buildings is system efficiency, environmental benefits and economic feasibility [5–7]. This is accomplished as a result of the utilization of the heat made available from electricity generation as well as elimination of losses due to transmission and distribution. It is especially meaningful to develop Microscale BCHP (MBCHP) system for the use of residential and light commercial buildings [8,9]. Kong et al. [10] finished the feasibility study and integration of a novel MBCHP system which can supply 12 kW electricity output and 28 kW heating output or 9 kW cooling output. The
Abbreviations: BCHP, building cooling heating and power; DES, distributed energy system; MBCHP, Micro-scale building cooling heating and power; temp, temperature; VFH, varying hot water flow rate with mass recovery in heating pattern; VTH, varying hot water inlet temperature with mass recovery in heating pattern; VTNH, varying hot water inlet temperature with mass recovery in no heating pattern * Corresponding author. Tel.: +86 21 6293 3250; fax: +86 21 6293 2601. E-mail address: [email protected] (J.Y. Wu). 0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.04.013
system is equipped with a LPG and natural gas engine generator set with heat recovery from the jacket water and exhaust gas, and a new adsorption chiller which is able to utilize low-grade heat source with the temperature less than 85 8C [11,12]. In summer, the MBCHP system supplies cooling, heating and electricity simultaneously for the demand side. The recovered heat, which meets the demand of heat consumer and adsorption chiller, changes with the variation of electricity output. Generally, the interaction among the outputs of electricity, heating and cooling is very complex. For example, at the same electricity output, the heating power for adsorption chiller decreases as the thermal demand from heat consumer increases and thus the cooling output declines. Therefore, the adsorption chiller always runs under varying heating conditions and hence the cooling output changes with that. To regulate the heating power and cooling output of adsorption chiller effectively, it is advisable to adopt proper operation mode in response to the recovered heat from MBCHP system under different electricity output conditions. Generally, the commonly used regulation methods are: (1) varying hot water inlet temperature; (2) varying hot water flow rate. The two methods regulate the temperature and heat transfer coefficient in the hot water loop, respectively. In addition,
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Nomenclature Cp,w COP ˙ chilled G ˙h G Pel PER Qad Qf Qh Qref Tchilled, Tchilled, Th, in Th, out gelh hel Dt tcyc
specific heat of water at constant pressure (kJ/ (kg 8C)) coefficient of performance (dimensionless) chilled water flow rate (kg/s) hot water flow rate (kg/s) electricity power (kW) primary energy ratio (dimensionless) heating power of adsorption chiller (kW) energy input (kW) recovered thermal heat (kW) cooling output (kW) in inlet temperature of chilled water (8C) out outlet temperature of chilled water (8C) inlet temperature of hot water (8C) outlet temperature of hot water (8C) electricity to heat ratio (dimensionless) electricity efficiency (dimensionless) data sampling time interval (s) cycling time (s)
Subscripts ad adsorption chiller cyc cycle chilled chilled water el electricity f fuel h thermal heat in inlet out outlet p constant pressure ref refrigeration w water superscript i sampling time
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recovery in no heating pattern (VTNH) and varying hot water flow rate with mass recovery in heating pattern (VFH). 2. MBCHP system configuration and varying load characteristics 2.1. Description of MBCHP system MBCHP system is composed of internal combustion (I.C.) gas engine with 12 kW electricity output, adsorption chiller with 9 kW cooling output, heat exchange equipment, fan coil, floor heating system, cooling tower, water tank, pumps and connection pipes etc [10]. The schematic diagram of MBCHP system can be seen in Fig. 1. The fuel of I.C. gas engine is natural gas or LPG. The working pair of adsorption chiller is silica gel/water and the driving temperature is from 65 to 85 8C. MBCHP system recovers the heat from jacket water and exhaust gas in the power generation process, and uses the recovered heat to provide floor heating in winter, drive the adsorption chiller for cooling in summer and supply domestic hot water all over the year. This MBCHP system can meet the demand of heating, cooling and domestic hot water of the building area over 100 m2. According to the functions, the whole system is divided into the following units: power generation unit, energy conversion unit, cooling supply unit, domestic hot water supply unit, floor heating unit, and electricity supply unit. 2.2. The load varying characteristics of MBCHP system The data of electricity and heating output are obtained from detailed gas engine experiments. To evaluate system performance, the following functions are adopted: Electricity to heat ratio: g elh ¼
Pel Qh
(1)
Electricity efficiency: hel ¼ according to the heating status of adsorber, the patterns of mass recovery process can be divided into: (1) the mass recovery process in heating pattern; (2) the mass recovery process in no heating pattern. In theory, cooling output is high with mass recovery in heating pattern for the strong effect of ‘‘second desorption’’ and the value of COP is high with mass recovery in no heating pattern for the effective utilization of sensible heat of adsorber [11,12]. To guide the optimum matching and operation of adsorption chiller for MBCHP system application correctly, it is advisable to make clear the influence on adsorption chiller performance by different operation modes under varying heating conditions. This paper focuses on the study of the performance of adsorption chiller in the following operation modes: varying hot water inlet temperature with mass recovery in heating pattern (VTH), varying hot water inlet temperature with mass
Pel Qf
(2)
Primary energy ratio: PER ¼
Pel þ Qh Qf
(3)
where Qh, Pel and Qf are the power of recovered heat, electricity output and energy input, respectively. gelh, hel and PER are electricity to heat ratio, electricity efficiency and primary energy ratio, respectively. Electricity to heat ratio and electricity efficiency as function of electricity output is shown in Fig. 2. It can be stated that the values of electricity efficiency and electricity to heat ratio are low and have sharp increase with the electricity output range from 0.0 to 6.0 kW. After that, electricity efficiency and electricity to heat ratio reach high level and increase slightly. It indicates that the MBCHP system runs economically with the electricity output surpassing
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Fig. 1. Schematic diagram of MBCHP system.
50% of full electricity load. Therefore, the following discussed electricity output conditions of MBCHP system are all above 6.0 kW. Fig. 3 shows recovered heat and the value of PER as function of electricity output. It is seen that the recovered heat increases from 17.9 to 28.1 kW with the electricity range from 6 to 12 kW. Hence, the available heating output from the MBCHP system varies with the change of electricity output conditions, which requires good varying load characteristics of the adsorption chiller. To make the heating power of adsorption chiller match the recovered heat and run with high COP, the following regulation methods are usually considered: varying hot water inlet temperature, varying hot water flow rate, varying cycle time and varying mass recovery pattern.
Fig. 2. Electricity to heat ratio and electricity efficiency vs. electricity output.
3. Experimental investigation of adsorption chiller performance under varying heating conditions 3.1. Test system introduction To simplify investigation and obtain accurate operation data of adsorption chiller under varying heating conditions, test system is established and experiments are carried out. The test system, as shown in Fig. 4, is composed of adsorption chiller, hot water loop, cooling water loop and chilled water loop. Detailed information about the structure and operation principle of the adsorption chiller can be referred to [11]. Each water loop is equipped with a water tank to realize the function of regulating inlet temperature and buffering temperature fluctuation. The volumes of hot water tank, cooling water tank and chilled water
Fig. 3. Recovered heat and PER vs. electricity output.
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clear the influence of different operation modes of VTH, VTNH and VFH. The detailed experimental conditions are listed in Table 2. The experimental work continues under different working conditions. So no less than four cycles is repeated for every working condition in order to slake the influence of the former working conditions on the present. All the data in this paper is obtained from the fourth cycle. Cooling output is calculated as follows: R tcyc ˙ chilled Cp;w ðTchilled;in Tchilled;out Þ dt G Qref ¼ 0 tcyc P i i ˙ chilled Cp;w ðTchilled;in G Tchilled;out Þ ¼ Dt (4) tcyc
Fig. 4. Test system of adsorption chiller under varying heating conditions.
tank are 0.7, 1.0 and 1.0 m3, respectively. Valves V7, V5 and V4 control the inlet temperatures of hot water, cooling water and chilled water, respectively. Valves V2, V3 and V1 control the flow of hot water, cooling water and chilled water, respectively. The cooling water from the adsorber flows into the cooling water tank and is mixed with the chilled water in the chilled water tank. The redundant water is drained out of the cooling water tank to spray drain through an overflow pipe. The nominal flow rates of hot water, cooling water and chilled water are 1.0, 1.3 and 0.5 kg/ s, respectively. The inlet temperature fluctuations are controlled within 1.5, 0.5 and 0.5 8C for hot water, cooling water and chilled water, respectively. In this study, the hot water inlet temperature is controlled in a range from 65 to 85 8C. The reason is that the temperature in the water loop of MBCHP system should not be too high (>85 8C) or too low (<65 8C). If the temperature is too high, it will affect the safe operation of gas engine and damage the silica gel. On the contrary, it will shorten the operation life of gas engine and cause uneconomic operation of adsorption chiller if the temperature is too low in a long operation time. In addition,the inlet temperatures of cooling water and chilled water are controlled at 32 and 20 8C, respectively. The mass recovery time is kept at 180 s. The sensors and measurements employed in the test system are listed in Table 1. 3.2. Experimental methods and performance coefficients calculations The main aims of this work are to test the performance of the adsorption chiller under varying heating conditions and make
˙ chilled and Cp,w are cooling output, chilled water where Qref, G flow rate and water specific heat, respectively. Tchilled, in and Tchilled, out are the inlet and outlet temperatures of chilled water, respectively. Dt is the sampling time interval and tcyc is the cycle time. Superscript i is sampling time i. Heating power of adsorption chiller: R tcyc ˙ h Cp;w ðTh;in Th;out Þdt G Qad ¼ 0 tcyc P i i ˙ h Cp;w ðTh;in Dt G Th;out Þ ¼ (5) t cyc and COP: COP ¼
Qref Qad
(6)
˙ h is where Qad is the heating power of adsorption chiller and G the hot water flow rate. Th,in and Th,out are the inlet and outlet temperatures of hot water, respectively. 4. Experimental results and analysis 4.1. Adsorption chiller performance under varying heating conditions 4.1.1. Performance in VTH mode The effect of the hot water inlet temperature on heating power and COP in VTH mode is shown in Fig. 5. It is evident that the heating power of adsorption chiller varies almost linearly with the hot water inlet temperature, which is good for regulation. With the hot water temperature range from 65 to 85 8C, the heating power of adsorption chiller changes from 16.2 to 21.9 kW, which matches the recovered heat of MBCHP system with the electricity output range from 6.0 to 12.0 kW.
Table 1 Employed sensors and instruments Function
Sensors or instruments
Accuracy
Water temperature measurement Water volumetric flow rate measurement
Pt-100 (4-wire measurement) Revolving flow meter
Data acquisition
Keithley 2700 multi-meter/data acquisition instrument
Grade A (100 V 0.10% at 0 8C) 0.5% accuracy in a range of 0–6 m3/h, with 4–20 mA output 6.5 bit
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Table 2 Description of experimental conditions Hot water flow rate (kg/s)
Hot water inlet temp (8C)
Cooling water inlet temp (8C)
Chilled water inlet temp (8C)
Cycle time (s)
Mass recovery time (s)
Mass recovery pattern
1.0 0.75 0.50 0.33 0.17 1.0 0.75 0.50 0.33 0.17 1.0
85/75/70/65 85/75 85/75 85/75 85/75 85 85 85 85 85 85/75/70/65
32 32 32 32 32 32 32 32 32 32 32
20 20 20 20 20 20 20 20 20 20 20
900 900 900 900 900 1000/1100/1200 1000/1100/1200 1000/1100/1200 1000/1100/1200 1000/1100/1200 900
180 180 180 180 180 180 180 180 180 180 180
Heating Heating Heating Heating Heating Heating Heating Heating Heating Heating No heating
4.1.3. Performance in VFH mode Fig. 7 shows the varying characteristics of heating power and COP with the change of hot water flow rate in VFH mode. The inlet temperatures of hot water are controlled at 85 and 75 8C, respectively. It can be stated that the heating power and COP vary parabolicly with the hot water flow rate. With the hot water flow rate range from 1 to 0.5 kg/s, the heating power and COP change within the range of 3.0 and 6.0%, respectively.
Therefore, the regulation performance is not good in this range of hot water flow rate. On the other hand, it is advisable that the hot water pump could be chosen smaller to save electricity consumption for the MBCHP system itself. The heating power and COP change rapidly with hot water flow rate range from 0.5 to 0.17 kg/s. The performance of adsorption chiller decreases more rapidly when the hot water inlet temperature is lower. For instance, with the hot water flow rate of 0.17 kg/s, the value of COP is 0.31 at 85 8C and drops to 0.26 at 75 8C. Therefore, it will cause uneconomic operation at low inlet temperature with low water flow less than 0.5 kg/s. It is advisable for adsorption chiller to run at high temperature of 85 8C under varying hot water flow rate conditions. In fact, the heat transfer characteristics in adsorber vary greatly when the hot water flow rate changes. Therefore, the corresponding optimum cycle time also changes. To obtain optimum cycle time at different hot water flow rate, experiments of varying cycle time are carried out. Some of the experimental data are shown in Fig. 8. The graph shows the cooling output and COP as function of cycle time in different hot water flow rate at 85 8C. It is obvious that the value of COP increases slightly with the rising of cycle time and the variation is within 8.0% from 900 to 1200 s. The optimum cycle time of maximum cooling output increases with the decrease of hot water flow rate. For example, the optimum cycle times are 1100 and 1200 s at 0.25 and
Fig. 5. Heating power and COP vs. hot water inlet temperature in VTH mode.
Fig. 6. Heating power and COP vs. hot water inlet temperature in VTNH mode.
The value of COP is kept at high levels, which are 0.43 and 0.37 with the hot water inlet temperature of 85 and 65 8C, respectively. COP changes slightly with hot water inlet temperature range from 75 to 85 8C and the variation is only within 4.0%. 4.1.2. Performance in VTNH mode Heating power and COP as function of hot water inlet temperature in VTNH mode is shown in Fig. 6. There is also an approximate linear relation between heating power and hot water inlet temperature in VTNH mode. Compared with the performance in VTH mode, it enjoys lower heating power and higher COP in VTNH mode and the advantage is more obvious with lower hot water inlet temperature of 65 8C. For example, the value of COP is as high as 0.40 and heating power is 2.0 kW lower in VTNH mode than in VTH mode at 65 8C.
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Fig. 7. Heating power and COP vs. hot water flow rate in VFH mode.
0.17 kg/s, respectively. The following discussion about the performance at different hot water flow rate refers to the conditions in the optimum cycle time with hot water inlet temperature of 85 8C. 4.2. Comparison of available heating and cooling output of MBCHP system in different adsorption chiller operation modes Since the heating power and COP of adsorption chiller varies in different operation modes, it will influence the available heating and cooling output of MBCHP system. In this paper, available heating output of MBCHP system indicates the remained heating output after meeting the thermal h eat demand from the adsorption chiller. 4.2.1. Comparison of available heating and cooling output in VTH and VFH mode Figs. 9 and 10 show the interrelation of available heating and cooling output as function of electricity output in VTH and VFH mode. The areas enclosed by solid line and dotted line indicate the regulation range in VTH and VFH mode, respectively. From Fig. 9, it can be seen that the regulation range of available cooling output is larger in VTH mode than in VFH
Fig. 8. Heating power and cooling output vs. cycle time.
Fig. 9. Comparison of regulation range of available cooling output vs. electricity output in VTH mode and VFH mode.
mode. For instance, with the electricity output of 7.5 kW, cooling output ranges from 6.0 to 8.3 kW in VTH mode and from 6.3 to 7.2 kW in VFH mode. When the electricity output is over 8.3 kW, the available cooling output reaches 9.4 kW in both modes. In VFH mode, there is no available cooling output with the electricity output less than 7.0 kW. The reason is that the recovered heat from MBCHP system is relatively low in the electricity output range from 6.0 to 7.0 kW and not enough to drive the adsorption chiller even at small hot water flow rate of 0.17 kg/s in VFH mode. Fig. 10 shows that the regulation range of available heating output is larger in VTH mode than in VFH mode with electricity output range from 6.0 to 12.0 kW. For example, with the electricity output of 12.0 kW, available heating output is from 6.1 to 8.5 kW in VTH mode and from 6.1 to 11.9 kW in VFH mode. 4.2.2. Comparison of available heating and cooling output in VTH and VTNH mode The interrelation of available heating and cooling output as function of electricity output in VTH mode and VTNH mode is shown in Figs. 11 and 12. The areas enclosed by solid line and
Fig. 10. Comparison of regulation range of available heating output vs. electricity output in VTH mode and VFH mode.
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Fig. 11. Comparison of regulation range of available cooling output vs. electricity output in VTH mode and VTNH mode.
dotted line indicate the regulation range in VTNH mode and VTH mode, respectively. Fig. 11 shows that the regulation range of available cooling output is larger in VTNH mode than in VTH mode with the electricity range from 6.0 to 8.3 kW. For instance, with the electricity output of 6.0 kW, cooling output is from 5.6 to 7.7 kW in VTNH mode and from 6.0 to 6.7 kW in VTH mode. The reason is that the recovered heat from gas engine is not high enough to drive the adsorption chiller run at 85 8C under the low electricity output conditions from 6.0 to 8.3 kW. Since VTNH mode is with higher COP and lower heating power, the hot water inlet temperature is higher in VTNH mode than in VTH mode at the same electricity output condition. For example, in response to the fitted results from experimental data, the hot water inlet temperature is respectively 67.4 8C in VTH mode and 75.1 8C in VTNH mode at 6 kW electricity output. Hence, cooling output is higher in VTNH mode with the electricity output range from 6.0 to 8.3 kW. When the electricity output is over 8.3 kW, the recovered heat is high enough to drive the adsorption chiller to run at 85 8C and the maximum cooling output is higher in VTH mode than in VTNH
Fig. 12. Comparison of regulation range of available heating output vs. electricity output in VTH mode and VTNH mode.
mode. For example, the maximum cooling output is 0.3 kW more in VTH mode than in VTNH mode. Fig. 12 shows that the maximum available heating output is larger in VTNH mode than in VTH mode with the electricity output range from 6.0 to 12.0 kW. For example, with the electricity output of 12.0 kW, the maximum available heating output is 2 kW more in VTNH mode than in VTH mode. From the discussed above, it can be stated that VTNH mode enjoys larger regulation area of cooling output in the low electricity conditions from 6.0 to 8.3 kW. Hence, when the cooling demand is with priority, it is advisable to adopt VTNH mode with the electricity output from 6.0 to 8.3 kW and VTH mode or VFH mode is preferred with the electricity output over 8.3 kW. The available heating output is highest in VTNH mode with electricity output range from 6.0 to 12.0 kW. Therefore, it is recommended to take VTNH mode when the heating demand is with priority. 5. Conclusions In the application of MBCHP system, there are complicated relations among the output of electricity, heating and cooling. The adsorption chiller often runs under varying heating conditions. To guide the optimum matching and operation of the adsorption chiller in MBCHP system correctly, experiments are carried out to make clear the adsorption chiller performance in VTH, VTNH and VFH mode. Through analysis of the experimental data, conclusions can be drawn as follows: (1) The heating power of adsorption chiller varies almost linearly with the change of hot water inlet temperature in VTH and VTNH mode, which is good for regulation. (2) Compared with VTH mode, VTNH mode has lower heating power and higher COP and the advantage is more obvious at lower hot water inlet temperature. Heating power is 2 kW lower in VTNH mode than in VTH mode at 65 8C. (3) In VFH mode, heating power of adsorption chiller and COP vary parabolicly with the hot water flow rate. The regulation performance is not good with the hot water flow rate range from 1 to 0.5 kg/s. Pump flow of hot water can be chosen smaller in this flow range to save electricity consumption of MBCHP system without refrigeration performance decreasing obviously. The heating power and COP change rapidly with the hot water flow rate range from 0.5 to 0.17 kg/s. It is advisable for adsorption chiller to run at high temperature of 85 8C under varying hot water flow rate conditions. The optimum cycle time of maximum cooling output increases with the decrease of hot water flow rate. (4) The regulation range of available cooling and heating output is larger in VTH mode than in VFH mode with the electricity output range from 6.0 to 12.0 kW. (5) From 6.0 to 8.3 kW of electricity output, the regulation range of available cooling and heating output is larger in VTNH mode than in VTH mode. With the electricity output range from 6.0 to 12.0 kW, the maximum heating output is larger in VTNH mode than in VTH mode.
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(6) When the cooling demand is with priority, it is advisable to adopt VTNH mode with the electricity output range from 6.0 to 8.3 kW and take VTH mode or VFH mode from 8.3 to 12.0 kW. With electricity output from 6.0 to 12.0 kW, VTNH mode is preferred when the heating demand is with priority. Acknowledgements This work was supported by the Special Fund of Higher Education Doctorate Subject under the contract No. 20040248055, Shanghai Commission of Science & Technology Special Fund for the 2010 World Expro under the contract No. 2005BA908B07. References [1] Hui Li, Lin Fu, Kecheng Geng, Yi Jiang, Energy utilization evaluation of CCHP systems, Energy and Buildings 38 (2006) 253–257. [2] R.Z. Wang, Some discussions on energy efficiency in building and hybrid energy systems, Acta Energiae Solaris Sinica 23 (2002) 322–335. [3] X.Q. Kong, R.Z. Wang, X.H. Huang, Energy efficiency and economic feasibility of CCHP driven by stirling engine, Energy Conversion and Management 45 (2004) 1433–1442.
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[4] V. Havelsky, Energetic efficiency of cogeneration systems for combined heat, cold and power production, International Journal of Refrigeration 22 (1999) 479–485. [5] E. Bilgen, Exergetic and engineering analyses of gas turbine based cogeneration systems, Energy 25 (2000) 1215–1229. [6] G.G. Maidment, R.M. Tozer, Combined cooling heat and power in supermarkets, Applied Thermal Engineering 22 (2002) 653–665. [7] C. Cormio, M. Dicorato, A. Minoia, M. Trovato, A regional energy planning methodology including renewable energy sources and environmental constraints, Renewable and Sustainable Energy Reviews 7 (2003) 99–130. [8] M. Dentice, D. Accadia, M. Sasso, Micro-combined heat and power in residential and light commericial applications, Applied Thermal Engineering 23 (2003) 1247–1259. [9] G. Gigliucci, L. Petruzzi, E. Cerelli, A. Garzisi, A. La Mendola, Demonstration of a residential CHP system based on PEM fuel cells, Journal of Power Sources 131 (2004) 62–68. [10] X.Q. Kong, R.Z. Wang, J.Y. Wu, X.H. Huang, Integeration and Experimental Investigation of a Micro-Combined Cooling, Heating and Power System, Journal of Engineering Thermophysics 26 (2005) 21–24. [11] D.C. Wang, J.Y. Wu, Z.Z. Xia, H. Zhai, R.Z. Wang, W.D. Dou, Study of a novel silica gel-water adsorption chiller. Part I. Design and performance prediction, International Journal of Refrigeration 28 (2005) 1073–1083. [12] D.C. Wang, J.Y. Wu, Z.Z. Xia, H. Zhai, R.Z. Wang, W.D. Dou, Study of a novel silica gel-water adsorption chiller. Part I. Experimental study, International Journal of Refrigeration 28 (2005) 1084–1091.
Experimental investigation of adsorption chiller for Micro-scale BCHP system application Y. Huangfu, J.Y. Wu *, R.Z. Wang, Z.Z. Xia Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China Received 5 March 2006; received in revised form 3 April 2006; accepted 4 April 2006
Abstract Building cooling, heating and power (BCHP), is an attracting cogeneration system for its economic and environmental friendly qualities. Therefore, it is meaningful to develop Micro-scale BCHP (MBCHP) system based on adsorption chiller and internal combustion gas engine for the use of residential and light commercial buildings. To guide the optimum matching and operation of adsorption chiller in MBCHP system correctly, this paper deals with the performance of adsorption chiller under varying heating conditions. Experimental results show that the value of COP is high in the operation mode of varying hot water inlet temperature with mass recovery in no heating pattern (VTNH). With the hot water inlet temperature of 65 8C, the value of COP is as high as 0.40 in VTNH mode. Under electricity output conditions from 6.0 to 8.3 kW in MBCHP system, VTNH mode is especially preferred when cooling demand is with priority. # 2006 Elsevier B.V. All rights reserved. Keywords: Adsorption chiller; MBCHP; Varying heating conditions; Experimental investigation; Application
1. Introduction As a kind of distributed energy system (DES), building cooling, heating and power (BCHP) is attracting more and more concerns [1–4]. The advantage of BCHP system for buildings is system efficiency, environmental benefits and economic feasibility [5–7]. This is accomplished as a result of the utilization of the heat made available from electricity generation as well as elimination of losses due to transmission and distribution. It is especially meaningful to develop Microscale BCHP (MBCHP) system for the use of residential and light commercial buildings [8,9]. Kong et al. [10] finished the feasibility study and integration of a novel MBCHP system which can supply 12 kW electricity output and 28 kW heating output or 9 kW cooling output. The
Abbreviations: BCHP, building cooling heating and power; DES, distributed energy system; MBCHP, Micro-scale building cooling heating and power; temp, temperature; VFH, varying hot water flow rate with mass recovery in heating pattern; VTH, varying hot water inlet temperature with mass recovery in heating pattern; VTNH, varying hot water inlet temperature with mass recovery in no heating pattern * Corresponding author. Tel.: +86 21 6293 3250; fax: +86 21 6293 2601. E-mail address: [email protected] (J.Y. Wu). 0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.04.013
system is equipped with a LPG and natural gas engine generator set with heat recovery from the jacket water and exhaust gas, and a new adsorption chiller which is able to utilize low-grade heat source with the temperature less than 85 8C [11,12]. In summer, the MBCHP system supplies cooling, heating and electricity simultaneously for the demand side. The recovered heat, which meets the demand of heat consumer and adsorption chiller, changes with the variation of electricity output. Generally, the interaction among the outputs of electricity, heating and cooling is very complex. For example, at the same electricity output, the heating power for adsorption chiller decreases as the thermal demand from heat consumer increases and thus the cooling output declines. Therefore, the adsorption chiller always runs under varying heating conditions and hence the cooling output changes with that. To regulate the heating power and cooling output of adsorption chiller effectively, it is advisable to adopt proper operation mode in response to the recovered heat from MBCHP system under different electricity output conditions. Generally, the commonly used regulation methods are: (1) varying hot water inlet temperature; (2) varying hot water flow rate. The two methods regulate the temperature and heat transfer coefficient in the hot water loop, respectively. In addition,
Y. Huangfu et al. / Energy and Buildings 39 (2007) 120–127
Nomenclature Cp,w COP ˙ chilled G ˙h G Pel PER Qad Qf Qh Qref Tchilled, Tchilled, Th, in Th, out gelh hel Dt tcyc
specific heat of water at constant pressure (kJ/ (kg 8C)) coefficient of performance (dimensionless) chilled water flow rate (kg/s) hot water flow rate (kg/s) electricity power (kW) primary energy ratio (dimensionless) heating power of adsorption chiller (kW) energy input (kW) recovered thermal heat (kW) cooling output (kW) in inlet temperature of chilled water (8C) out outlet temperature of chilled water (8C) inlet temperature of hot water (8C) outlet temperature of hot water (8C) electricity to heat ratio (dimensionless) electricity efficiency (dimensionless) data sampling time interval (s) cycling time (s)
Subscripts ad adsorption chiller cyc cycle chilled chilled water el electricity f fuel h thermal heat in inlet out outlet p constant pressure ref refrigeration w water superscript i sampling time
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recovery in no heating pattern (VTNH) and varying hot water flow rate with mass recovery in heating pattern (VFH). 2. MBCHP system configuration and varying load characteristics 2.1. Description of MBCHP system MBCHP system is composed of internal combustion (I.C.) gas engine with 12 kW electricity output, adsorption chiller with 9 kW cooling output, heat exchange equipment, fan coil, floor heating system, cooling tower, water tank, pumps and connection pipes etc [10]. The schematic diagram of MBCHP system can be seen in Fig. 1. The fuel of I.C. gas engine is natural gas or LPG. The working pair of adsorption chiller is silica gel/water and the driving temperature is from 65 to 85 8C. MBCHP system recovers the heat from jacket water and exhaust gas in the power generation process, and uses the recovered heat to provide floor heating in winter, drive the adsorption chiller for cooling in summer and supply domestic hot water all over the year. This MBCHP system can meet the demand of heating, cooling and domestic hot water of the building area over 100 m2. According to the functions, the whole system is divided into the following units: power generation unit, energy conversion unit, cooling supply unit, domestic hot water supply unit, floor heating unit, and electricity supply unit. 2.2. The load varying characteristics of MBCHP system The data of electricity and heating output are obtained from detailed gas engine experiments. To evaluate system performance, the following functions are adopted: Electricity to heat ratio: g elh ¼
Pel Qh
(1)
Electricity efficiency: hel ¼ according to the heating status of adsorber, the patterns of mass recovery process can be divided into: (1) the mass recovery process in heating pattern; (2) the mass recovery process in no heating pattern. In theory, cooling output is high with mass recovery in heating pattern for the strong effect of ‘‘second desorption’’ and the value of COP is high with mass recovery in no heating pattern for the effective utilization of sensible heat of adsorber [11,12]. To guide the optimum matching and operation of adsorption chiller for MBCHP system application correctly, it is advisable to make clear the influence on adsorption chiller performance by different operation modes under varying heating conditions. This paper focuses on the study of the performance of adsorption chiller in the following operation modes: varying hot water inlet temperature with mass recovery in heating pattern (VTH), varying hot water inlet temperature with mass
Pel Qf
(2)
Primary energy ratio: PER ¼
Pel þ Qh Qf
(3)
where Qh, Pel and Qf are the power of recovered heat, electricity output and energy input, respectively. gelh, hel and PER are electricity to heat ratio, electricity efficiency and primary energy ratio, respectively. Electricity to heat ratio and electricity efficiency as function of electricity output is shown in Fig. 2. It can be stated that the values of electricity efficiency and electricity to heat ratio are low and have sharp increase with the electricity output range from 0.0 to 6.0 kW. After that, electricity efficiency and electricity to heat ratio reach high level and increase slightly. It indicates that the MBCHP system runs economically with the electricity output surpassing
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Fig. 1. Schematic diagram of MBCHP system.
50% of full electricity load. Therefore, the following discussed electricity output conditions of MBCHP system are all above 6.0 kW. Fig. 3 shows recovered heat and the value of PER as function of electricity output. It is seen that the recovered heat increases from 17.9 to 28.1 kW with the electricity range from 6 to 12 kW. Hence, the available heating output from the MBCHP system varies with the change of electricity output conditions, which requires good varying load characteristics of the adsorption chiller. To make the heating power of adsorption chiller match the recovered heat and run with high COP, the following regulation methods are usually considered: varying hot water inlet temperature, varying hot water flow rate, varying cycle time and varying mass recovery pattern.
Fig. 2. Electricity to heat ratio and electricity efficiency vs. electricity output.
3. Experimental investigation of adsorption chiller performance under varying heating conditions 3.1. Test system introduction To simplify investigation and obtain accurate operation data of adsorption chiller under varying heating conditions, test system is established and experiments are carried out. The test system, as shown in Fig. 4, is composed of adsorption chiller, hot water loop, cooling water loop and chilled water loop. Detailed information about the structure and operation principle of the adsorption chiller can be referred to [11]. Each water loop is equipped with a water tank to realize the function of regulating inlet temperature and buffering temperature fluctuation. The volumes of hot water tank, cooling water tank and chilled water
Fig. 3. Recovered heat and PER vs. electricity output.
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clear the influence of different operation modes of VTH, VTNH and VFH. The detailed experimental conditions are listed in Table 2. The experimental work continues under different working conditions. So no less than four cycles is repeated for every working condition in order to slake the influence of the former working conditions on the present. All the data in this paper is obtained from the fourth cycle. Cooling output is calculated as follows: R tcyc ˙ chilled Cp;w ðTchilled;in Tchilled;out Þ dt G Qref ¼ 0 tcyc P i i ˙ chilled Cp;w ðTchilled;in G Tchilled;out Þ ¼ Dt (4) tcyc
Fig. 4. Test system of adsorption chiller under varying heating conditions.
tank are 0.7, 1.0 and 1.0 m3, respectively. Valves V7, V5 and V4 control the inlet temperatures of hot water, cooling water and chilled water, respectively. Valves V2, V3 and V1 control the flow of hot water, cooling water and chilled water, respectively. The cooling water from the adsorber flows into the cooling water tank and is mixed with the chilled water in the chilled water tank. The redundant water is drained out of the cooling water tank to spray drain through an overflow pipe. The nominal flow rates of hot water, cooling water and chilled water are 1.0, 1.3 and 0.5 kg/ s, respectively. The inlet temperature fluctuations are controlled within 1.5, 0.5 and 0.5 8C for hot water, cooling water and chilled water, respectively. In this study, the hot water inlet temperature is controlled in a range from 65 to 85 8C. The reason is that the temperature in the water loop of MBCHP system should not be too high (>85 8C) or too low (<65 8C). If the temperature is too high, it will affect the safe operation of gas engine and damage the silica gel. On the contrary, it will shorten the operation life of gas engine and cause uneconomic operation of adsorption chiller if the temperature is too low in a long operation time. In addition,the inlet temperatures of cooling water and chilled water are controlled at 32 and 20 8C, respectively. The mass recovery time is kept at 180 s. The sensors and measurements employed in the test system are listed in Table 1. 3.2. Experimental methods and performance coefficients calculations The main aims of this work are to test the performance of the adsorption chiller under varying heating conditions and make
˙ chilled and Cp,w are cooling output, chilled water where Qref, G flow rate and water specific heat, respectively. Tchilled, in and Tchilled, out are the inlet and outlet temperatures of chilled water, respectively. Dt is the sampling time interval and tcyc is the cycle time. Superscript i is sampling time i. Heating power of adsorption chiller: R tcyc ˙ h Cp;w ðTh;in Th;out Þdt G Qad ¼ 0 tcyc P i i ˙ h Cp;w ðTh;in Dt G Th;out Þ ¼ (5) t cyc and COP: COP ¼
Qref Qad
(6)
˙ h is where Qad is the heating power of adsorption chiller and G the hot water flow rate. Th,in and Th,out are the inlet and outlet temperatures of hot water, respectively. 4. Experimental results and analysis 4.1. Adsorption chiller performance under varying heating conditions 4.1.1. Performance in VTH mode The effect of the hot water inlet temperature on heating power and COP in VTH mode is shown in Fig. 5. It is evident that the heating power of adsorption chiller varies almost linearly with the hot water inlet temperature, which is good for regulation. With the hot water temperature range from 65 to 85 8C, the heating power of adsorption chiller changes from 16.2 to 21.9 kW, which matches the recovered heat of MBCHP system with the electricity output range from 6.0 to 12.0 kW.
Table 1 Employed sensors and instruments Function
Sensors or instruments
Accuracy
Water temperature measurement Water volumetric flow rate measurement
Pt-100 (4-wire measurement) Revolving flow meter
Data acquisition
Keithley 2700 multi-meter/data acquisition instrument
Grade A (100 V 0.10% at 0 8C) 0.5% accuracy in a range of 0–6 m3/h, with 4–20 mA output 6.5 bit
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Table 2 Description of experimental conditions Hot water flow rate (kg/s)
Hot water inlet temp (8C)
Cooling water inlet temp (8C)
Chilled water inlet temp (8C)
Cycle time (s)
Mass recovery time (s)
Mass recovery pattern
1.0 0.75 0.50 0.33 0.17 1.0 0.75 0.50 0.33 0.17 1.0
85/75/70/65 85/75 85/75 85/75 85/75 85 85 85 85 85 85/75/70/65
32 32 32 32 32 32 32 32 32 32 32
20 20 20 20 20 20 20 20 20 20 20
900 900 900 900 900 1000/1100/1200 1000/1100/1200 1000/1100/1200 1000/1100/1200 1000/1100/1200 900
180 180 180 180 180 180 180 180 180 180 180
Heating Heating Heating Heating Heating Heating Heating Heating Heating Heating No heating
4.1.3. Performance in VFH mode Fig. 7 shows the varying characteristics of heating power and COP with the change of hot water flow rate in VFH mode. The inlet temperatures of hot water are controlled at 85 and 75 8C, respectively. It can be stated that the heating power and COP vary parabolicly with the hot water flow rate. With the hot water flow rate range from 1 to 0.5 kg/s, the heating power and COP change within the range of 3.0 and 6.0%, respectively.
Therefore, the regulation performance is not good in this range of hot water flow rate. On the other hand, it is advisable that the hot water pump could be chosen smaller to save electricity consumption for the MBCHP system itself. The heating power and COP change rapidly with hot water flow rate range from 0.5 to 0.17 kg/s. The performance of adsorption chiller decreases more rapidly when the hot water inlet temperature is lower. For instance, with the hot water flow rate of 0.17 kg/s, the value of COP is 0.31 at 85 8C and drops to 0.26 at 75 8C. Therefore, it will cause uneconomic operation at low inlet temperature with low water flow less than 0.5 kg/s. It is advisable for adsorption chiller to run at high temperature of 85 8C under varying hot water flow rate conditions. In fact, the heat transfer characteristics in adsorber vary greatly when the hot water flow rate changes. Therefore, the corresponding optimum cycle time also changes. To obtain optimum cycle time at different hot water flow rate, experiments of varying cycle time are carried out. Some of the experimental data are shown in Fig. 8. The graph shows the cooling output and COP as function of cycle time in different hot water flow rate at 85 8C. It is obvious that the value of COP increases slightly with the rising of cycle time and the variation is within 8.0% from 900 to 1200 s. The optimum cycle time of maximum cooling output increases with the decrease of hot water flow rate. For example, the optimum cycle times are 1100 and 1200 s at 0.25 and
Fig. 5. Heating power and COP vs. hot water inlet temperature in VTH mode.
Fig. 6. Heating power and COP vs. hot water inlet temperature in VTNH mode.
The value of COP is kept at high levels, which are 0.43 and 0.37 with the hot water inlet temperature of 85 and 65 8C, respectively. COP changes slightly with hot water inlet temperature range from 75 to 85 8C and the variation is only within 4.0%. 4.1.2. Performance in VTNH mode Heating power and COP as function of hot water inlet temperature in VTNH mode is shown in Fig. 6. There is also an approximate linear relation between heating power and hot water inlet temperature in VTNH mode. Compared with the performance in VTH mode, it enjoys lower heating power and higher COP in VTNH mode and the advantage is more obvious with lower hot water inlet temperature of 65 8C. For example, the value of COP is as high as 0.40 and heating power is 2.0 kW lower in VTNH mode than in VTH mode at 65 8C.
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Fig. 7. Heating power and COP vs. hot water flow rate in VFH mode.
0.17 kg/s, respectively. The following discussion about the performance at different hot water flow rate refers to the conditions in the optimum cycle time with hot water inlet temperature of 85 8C. 4.2. Comparison of available heating and cooling output of MBCHP system in different adsorption chiller operation modes Since the heating power and COP of adsorption chiller varies in different operation modes, it will influence the available heating and cooling output of MBCHP system. In this paper, available heating output of MBCHP system indicates the remained heating output after meeting the thermal h eat demand from the adsorption chiller. 4.2.1. Comparison of available heating and cooling output in VTH and VFH mode Figs. 9 and 10 show the interrelation of available heating and cooling output as function of electricity output in VTH and VFH mode. The areas enclosed by solid line and dotted line indicate the regulation range in VTH and VFH mode, respectively. From Fig. 9, it can be seen that the regulation range of available cooling output is larger in VTH mode than in VFH
Fig. 8. Heating power and cooling output vs. cycle time.
Fig. 9. Comparison of regulation range of available cooling output vs. electricity output in VTH mode and VFH mode.
mode. For instance, with the electricity output of 7.5 kW, cooling output ranges from 6.0 to 8.3 kW in VTH mode and from 6.3 to 7.2 kW in VFH mode. When the electricity output is over 8.3 kW, the available cooling output reaches 9.4 kW in both modes. In VFH mode, there is no available cooling output with the electricity output less than 7.0 kW. The reason is that the recovered heat from MBCHP system is relatively low in the electricity output range from 6.0 to 7.0 kW and not enough to drive the adsorption chiller even at small hot water flow rate of 0.17 kg/s in VFH mode. Fig. 10 shows that the regulation range of available heating output is larger in VTH mode than in VFH mode with electricity output range from 6.0 to 12.0 kW. For example, with the electricity output of 12.0 kW, available heating output is from 6.1 to 8.5 kW in VTH mode and from 6.1 to 11.9 kW in VFH mode. 4.2.2. Comparison of available heating and cooling output in VTH and VTNH mode The interrelation of available heating and cooling output as function of electricity output in VTH mode and VTNH mode is shown in Figs. 11 and 12. The areas enclosed by solid line and
Fig. 10. Comparison of regulation range of available heating output vs. electricity output in VTH mode and VFH mode.
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Fig. 11. Comparison of regulation range of available cooling output vs. electricity output in VTH mode and VTNH mode.
dotted line indicate the regulation range in VTNH mode and VTH mode, respectively. Fig. 11 shows that the regulation range of available cooling output is larger in VTNH mode than in VTH mode with the electricity range from 6.0 to 8.3 kW. For instance, with the electricity output of 6.0 kW, cooling output is from 5.6 to 7.7 kW in VTNH mode and from 6.0 to 6.7 kW in VTH mode. The reason is that the recovered heat from gas engine is not high enough to drive the adsorption chiller run at 85 8C under the low electricity output conditions from 6.0 to 8.3 kW. Since VTNH mode is with higher COP and lower heating power, the hot water inlet temperature is higher in VTNH mode than in VTH mode at the same electricity output condition. For example, in response to the fitted results from experimental data, the hot water inlet temperature is respectively 67.4 8C in VTH mode and 75.1 8C in VTNH mode at 6 kW electricity output. Hence, cooling output is higher in VTNH mode with the electricity output range from 6.0 to 8.3 kW. When the electricity output is over 8.3 kW, the recovered heat is high enough to drive the adsorption chiller to run at 85 8C and the maximum cooling output is higher in VTH mode than in VTNH
Fig. 12. Comparison of regulation range of available heating output vs. electricity output in VTH mode and VTNH mode.
mode. For example, the maximum cooling output is 0.3 kW more in VTH mode than in VTNH mode. Fig. 12 shows that the maximum available heating output is larger in VTNH mode than in VTH mode with the electricity output range from 6.0 to 12.0 kW. For example, with the electricity output of 12.0 kW, the maximum available heating output is 2 kW more in VTNH mode than in VTH mode. From the discussed above, it can be stated that VTNH mode enjoys larger regulation area of cooling output in the low electricity conditions from 6.0 to 8.3 kW. Hence, when the cooling demand is with priority, it is advisable to adopt VTNH mode with the electricity output from 6.0 to 8.3 kW and VTH mode or VFH mode is preferred with the electricity output over 8.3 kW. The available heating output is highest in VTNH mode with electricity output range from 6.0 to 12.0 kW. Therefore, it is recommended to take VTNH mode when the heating demand is with priority. 5. Conclusions In the application of MBCHP system, there are complicated relations among the output of electricity, heating and cooling. The adsorption chiller often runs under varying heating conditions. To guide the optimum matching and operation of the adsorption chiller in MBCHP system correctly, experiments are carried out to make clear the adsorption chiller performance in VTH, VTNH and VFH mode. Through analysis of the experimental data, conclusions can be drawn as follows: (1) The heating power of adsorption chiller varies almost linearly with the change of hot water inlet temperature in VTH and VTNH mode, which is good for regulation. (2) Compared with VTH mode, VTNH mode has lower heating power and higher COP and the advantage is more obvious at lower hot water inlet temperature. Heating power is 2 kW lower in VTNH mode than in VTH mode at 65 8C. (3) In VFH mode, heating power of adsorption chiller and COP vary parabolicly with the hot water flow rate. The regulation performance is not good with the hot water flow rate range from 1 to 0.5 kg/s. Pump flow of hot water can be chosen smaller in this flow range to save electricity consumption of MBCHP system without refrigeration performance decreasing obviously. The heating power and COP change rapidly with the hot water flow rate range from 0.5 to 0.17 kg/s. It is advisable for adsorption chiller to run at high temperature of 85 8C under varying hot water flow rate conditions. The optimum cycle time of maximum cooling output increases with the decrease of hot water flow rate. (4) The regulation range of available cooling and heating output is larger in VTH mode than in VFH mode with the electricity output range from 6.0 to 12.0 kW. (5) From 6.0 to 8.3 kW of electricity output, the regulation range of available cooling and heating output is larger in VTNH mode than in VTH mode. With the electricity output range from 6.0 to 12.0 kW, the maximum heating output is larger in VTNH mode than in VTH mode.
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(6) When the cooling demand is with priority, it is advisable to adopt VTNH mode with the electricity output range from 6.0 to 8.3 kW and take VTH mode or VFH mode from 8.3 to 12.0 kW. With electricity output from 6.0 to 12.0 kW, VTNH mode is preferred when the heating demand is with priority. Acknowledgements This work was supported by the Special Fund of Higher Education Doctorate Subject under the contract No. 20040248055, Shanghai Commission of Science & Technology Special Fund for the 2010 World Expro under the contract No. 2005BA908B07. References [1] Hui Li, Lin Fu, Kecheng Geng, Yi Jiang, Energy utilization evaluation of CCHP systems, Energy and Buildings 38 (2006) 253–257. [2] R.Z. Wang, Some discussions on energy efficiency in building and hybrid energy systems, Acta Energiae Solaris Sinica 23 (2002) 322–335. [3] X.Q. Kong, R.Z. Wang, X.H. Huang, Energy efficiency and economic feasibility of CCHP driven by stirling engine, Energy Conversion and Management 45 (2004) 1433–1442.
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