OPTIMIZATION OF A GAS ABSORPTION HEAT PUMP SYSTEM

Transcription

1 OPTIMIZATION OF A GAS ABSORPTION HEAT PUMP SYSTEM J. L. CORRALES CIGANDA, R. GRAF, A. KÜHN, F. ZIEGLER Technische Universität Berlin, Department of Energy Engineering, KT 2, Marchstraße 18, 1587 Berlin, Germany, , jose.l.corralesciganda@tu-berlin.de ABSTRACT In the course of an energy-efficient modernization of a kindergarten in Berlin a 35 kw gas absorption heat pump using NH3/H2O as working pair has been installed in 21. Domestic hot water production and space heating are additionally supported by a 15 m 2 solar collector. The low temperature heat is supplied to the heat pump system by four 1 m long vertical ground probes. In this paper operation data for the period are presented. Gas Utilization Efficiencies (G.U.E.) are calculated and compared for different demand profiles. The comparison of measured G.U.E. values with manufacturer data revealed potential for system optimization. Based on operation analysis modifications of the control strategy and alternative hydraulic set-ups for an improved performance are presented. 1. INTRODUCTION In the medium capacity range of around 5kW, gas driven absorption heat pumps are the most efficient market available technology for combined hot water production (DHW) and space heating in terms of gas usage. Gas absorption heat pumps with NH3/H2O as working pair are mostly found in this capacity range. Monitoring results for heat pumps in these range presented measured seasonal gas utilization efficiencies (GUE) between 1.23 and 1.54, depending on the working conditions (Nitchske-Kowsky and Weißing, 211) (Moser and Rieberer, 211). At the Department of Energy Engineering of the Berlin Institute of Technology (TU Berlin) a project funded by the European Union and the federal state of Berlin started in July 211 aims to verify this energy saving potential and develop control strategies to optimise the heat pump performance. Three gas driven absorption systems working under different conditions (providing heat to a kindergarten, a swimming pool, and a social resident house) are subject to monitoring. Their performance will be analysed and compared with that of a laboratory test plant installed at TU Berlin. This work presents performance figures for the gas absorption heat pump working for space heating and domestic hot water preparation at the kindergarten. The performance under real working conditions and the influences of hydraulic set-up and control strategy are discussed and modifications for an optimised performance proposed. 2. HEATING SYSTEM AND CONTROL STRATEGY The key component of the heating system is a 35kW NH3/H2O condensing modulating heat pump optimised for high temperature. In a burner chamber within the heat pump gas is burned to provide input heat to a GAX-Cycle, and the exhausted gas is used in an additional heat exchanger (also included within the heat pump) to preheat the hot water before entering absorber and condenser (Ainardi and Guerra, 28). The hydraulic set up of the plant is presented in figure 1. The hot water produced at the heat pump flows into a 15L tank designed to store hot water for combined DHW production and space heating. This tank is divided into two sections, being the upper one at a higher temperature level for DHW preparation and a smaller lower part at a lower temperature for space heating. A 15m 2 solar collector field provides heat to the lower part of the combined storage tank. From this lower part of the tank hot water flows to the space heating circuit (two separated loops for space heating of an old 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213

2 and new section of the kindergarten). An additional buffer storage tank for space heating has been installed in parallel to the lower part of the combined tank being both charged with hot water from the gas heat pump at the same time. Four 1 m long vertical ground probes supply the low temperature heat to the heat pump and are a second renewable energy source for the heating system in addition to the solar collector field. 15m 2 solar collector field DHW Load cold water CONTROL SYSTEM Space Heating Load Old building New building HM solar V T T Tct1 V1 AB Heating system control THW2,SET Tsh1 COMBINED TANK A Tct2 B V2 Heat pump control EM Electric meter HM Heat meter T Temperature sensor b buffer ct combined tank gs ground source hw hot water sh space heating 1 top / in 2 bottom / out control signal Tsh2 Tb1 Tb2 BUFFER EM 2 V Thw2 Thw1 HM heat pump GAS HEAT PUMP EM 1 EM 3 GAS METER Figure 1: Heating system scheme with measurement instrumentation Tgs1 Tgs2 4x1m ground probes The heating system control switches ON/FF the heat pump and sends to its control unit a set value for the outlet hot water temperature value (T HW,out,SET ) to feed the buffer and/or the combined tank. The decision to start/stop operation for DHW preparation or space heating depends on the temperatures measured at the combined and buffer tank respectively. The system considers that the domestic hot water tank (upper part of the combined one) must be heated up if their temperature at both its top (T CT1 ) and its bottom (T CT2 ) is below 6 C. In this case the heat pump is switched on with a set value (T HW,out,SET ) of 7 C. The heat pump operates with this set value until the temperature at the bottom of the upper part of the combined tank (T CT2 ) is above 61 C. In this DHW preparation mode the 3-way valve V1 is set to position AB A and the valve V2 is opened. A heating curve calculates the hot water temperature required for space heating depending on the ambient temperature. This value is passed as T HW,out,SET to the heat pump if the ambient temperature is below 25 C (space heating operation mode). In this case the heat pump starts working if the temperature at top of the buffer (T B1 ) is below the set point calculated with the heating curve and terminates operation when the temperature at the lower level (T B2 ) has achieved a certain maximum level. For space heating operation the 3 way-vale V2 is set to AB B and the valve V2 is closed. If the conditions for both DHW preparation and space heating modes are present the system will work in DHW preparation mode. 3. MEASUREMENT INSTRUMENTATION AND UNCERTAINITIES Heat meters have been installed to quantify the heat production of the solar collector field and the heat pump. Electric meters have also been installed to measure the electric consumption of the heat pump, including that of the hot water and ground source pumps and the internal consumption of the heat pump itself (mainly control, burner air fan, and solution pump). In addition to these meters, the data logger records all relevant temperatures registered and used by the heating system control unit and other system relevant temperatures. Most of these recorded temperatures are shown in figure 1. The data logger also register values of the heat pump control 2 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213

3 unit including some internal temperature values and other working parameters (solution pump RPM, module status, etc). Acquired data is stored with a one minute interval excepting heat meter measurements that can be read and saved only every 15 minutes. The gas consumption is measured with a diaphragm gas flow meter. This meter was already present previous to the heating system modernization, and the model installed does not allow automatic data logging. For this reason gas measurements are based on regular readings of kindergarten personal. All external temperature sensors are of type PT-1. The heat meters belong to metrology class 3 according to the norm EN1434, the electric meters to class B according to EN 547, and the gas meter to the class 1.5 according to EN All measurement uncertainties are calculated following these standards. Table 1 summarises the measurement equipment uncertainties. Table 1: measurement equipment, heating value and measurement conditions uncertainties u(h s,n ) u(t gas ) u(p atm ) u(p gas ) u(p w ) u(v gas ) u(q HW / Solar ) u(e) 1% 5K 3% 3% 3% 3% 5,2% +2kWh 1% Table 1 also includes uncertainties for the heating value of the gas and the conditions present during gas measurements. The upper heating value of the gas under standard conditions (H s,n ) for each measured period has been obtained from the local gas supplier. Atmospheric pressure (p atm ) data from a nearby weather station run by the Federal German Weather Agency is used and a value of 14 C for the gas temperature (T gas ) has been derived from the yearly average conversion factor used by the gas supplier. The gas pressure (p gas ) has been assumed to be 2 mbar (since it is controlled via a pressure controller) and the partial pressure of steam contained in the gas (p w ) to be 2.17 mbar. Data monitoring started in October 21, and first available gas measurements are from April SEASONAL PERFORMANCE FIGURES With the measured values from heat meters it is possible to plot the contribution of solar collector field and heat pump to the total heat production of the system for each month of 211 and 212. Heat Input in MWh No Data availabe for 212 until March HP 211 Solar 211 HP 212 Solar Solar collectors stop working 4 2. Jan Feb March April Mai Juni Juli Aug Sept Okt Nov Dez Figure 2: Total heat production of solar collector field and heat pump Figure 2 reveals that the solar collector field can cover to 5% of the heating demand during summer months. For unknown reasons, the collector field stopped working at the end of June 212 (the reasons for this being under investigation). Although due to acquisition problems the first two months of 212 have not been analysed yet, figure 2 shows similar production profiles for both years. It is also seen that the heat provided for the months between November and March represents over 75% of the total heat supplied. Consequently system performance during these months will have a much bigger impact in the yearly performance of the system than that of the summer months. 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213 3

4 For evaluation of the energy efficiency of the heat pump, a seasonal Gas Utilization Efficiency (GUE seas ) will be used. This value is defined based on the standard EN In this standard gas volume is assumed to be at a reference state of 1 atm and 15 C. Consequently also the lower heating value (H i ) is to be converted to these conditions. Additionally, an auxiliary energy factor (AEF seas ) is defined to estimate the amount of electric power needed. It takes into account the internal electrical consumption of the heat pump (E HP,int ) and that of the ground source (E GS ) and hot water (E HW ) pumps. Values for GUE and AEF with gas measurements starting April 212 are shown in figure 3. GUE Year 212 AEF 5 4 Year April-Mai Mai-Juni Juni-Juli Juli-Aug Aug-Okt Okt-Nov Nov-Dez Dez-Jan 2 1 No electric meter April-Mai Mai-Juni Juni-Juli Juli-Aug Figure 3: GUE and AEF values for 212 Figure 3 presents better values for both thermal and electrical efficiencies for colder months. The GUE values obtained for most measured periods are around 1.2, going down to 1.14 between June and July and being almost 1.3 for the measured period between April and May. The measurement uncertainties however, do not allow observing any significant differences between the obtained GUE values. The uncertainties have been estimated assuming the previously presented values and the Gaussian error propagation as recommended by ISO (1995). The uncertainties are dominated by the metrology class of the heat meter and gas meter used. Using higher precision meters an uncertainty of around.4-.5 could be achieved. The calculated AEF values show a good electrical efficiency for the winter period and too high specific electric consumption for the months between July and October. During all these months the hot water and ground source pumps and the heat pump are in operation even if no heat is produced. This problem originates from the applied control strategy and is further investigated in section 5. Aug-Okt 5. PERFORMANCE ANALYSIS AND DISCUSSION In this chapter the heat pump and heating system performance are analysed, discussing how the first is affected by the latter. 5.1 Heat pump performance analysis All obtained GUE values are in a range between regardless of summer or winter. In summer however, with the heating curve setting lower heating water temperatures and the Okt-Nov Nov-Dez Dez-Jan 4 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213

5 temperature of the ground source being higher than in winter, a better thermal performance of the heat pump would be expected. On the other hand, measured GUE values for each monthly period are significantly below those reported by the manufacturer for the heat pump working with comparable hot water and ground source temperatures as table 2 shows. Table 2: heat pump performance figures from measurements and data catalogue Measured Period Activity T avg, HW,out T avg GS,in Q avg GUE seas Factor [ C] [ C] [kw] [-] Manufacturer data Manufacturer data December 212, 3h ±.41 Dec January , ±.1 June - July ±.11 April - May ±.11 The first two rows in Table 2 contain information taken from manufacturer certificates (Robur, 212). These data refer to laboratory measurements under steady state condit ions. In order to have a comparable experimental situation one quasi-stationary operation period of 3 hours for DHW preparation has been analysed and is presented in the third row. The last three rows in Table 2 present values obtained from some of the measurement periods presented in figure 3. Th e first column in this table show calculated values for the Activity Factor, defined as the ratio of heat pump operation time over total measured time. This factor has been introduced to quantify the discontinous operation of the heat pump. The factor is low for measurement periods with high ambient temperature, with low and non-frequendisplay average values for the hot water outlet and ground source inlet temperatures for each space heating demand. Second and third columns measured period and comparable temperatures specified by the manufacturer. The average outlet hot water temperatures are over 6 C in winter and below 4 C in summer/spring periods. The average inlet temperature from the ground source is below 7 C in winter and above 15 C in spring/summer. Second and third column show the temperatures levels of the heat sink and low temperature heat source. The temperature level of the high temperature heat source it is not measured, but the manufacturer reports values around 19 C for full load (Ainardi and Guerra, 28). If the gas burner is modulated this temperature can be reduced. The fourth column presents average heating capacities (considering for the average only the subperiods with heat pump operation) and those heating capacities specified by the manufacturer. For the 3h quasi-stationary operation measurement the value is comparable to that specified by manufacturer. However for all other three measured periods the average heating capacity is just a fraction of that value. The reason for this differences are two: the start/stop operation of the heating pump and its heat input modulation via the gas burner to maintain a given outlet hot water temperature as it is discussed more in detail within this work. The last column presents steady state GUE values from manufacturer data and seasonal GUE values for each measured period. Due mainly to the heat meter resolution the result for the 3h quasi-stationary measurement is not very accurate but within the error band it fits to the manufacturer's data. GUE values for longer measurement have a better accuracy and as mentioned before within the error band are in a similar range. The results from Table 2 suggest that the low activity frequency and the heat input modulation contribute to the difference between GUE values obtained for seasonal operation and those measured under nominal conditions. Regarding the discontinuous operation, similar patterns have been observed for summer operation 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213 5

6 of comparable systems (Moser and Rieberer, 211). For such operation patterns large thermal losses are expected. Discontinous operation is also responsible for the high specific electric consumption and low AEF values shown in Figure 2. Due to system control, both hot water and ground source pump are active even if the heat pump is not supplying any heat to the system. The gas burner (input heat source) modulation seems to have also an impact in the heat pump efficiency. Manufacturer catalogue data just provide information about the heat pump performance for full burner capacity. However according to measurements reported for certification under stationary standard conditions, the heat pump GUE seems to decrease significantly for low part load operation (Robur, 212). Future laboratory measurements at TU Berlin will be performed in order to analyse the performance of the heat pump with a significant burner modulation. 5.2 System operation analysis Figure 4 shows measured and set temperatures at the heat pump, solution pump status (ON/OFF) and burner modulation levels. 8 8 T e m p e r a tu r e in C ,5 T HW,out T HW,in T GS,IN T set Tem perature in C T HW,in T HW,out T GS,OUT 1 power modulation,5 Solution pump status power modulation Solution pump status 6: 12: 18: : 6: 12: 18: : 15/Jul/212 15/Dez/212 Figure 4: HP temperature profiles for a day in summer (left) and winter (right) For a summer day operation (Figure 4 left) the temperature of the ground probes is ca. 1 C during operation rising to over 2 C with no heat demand (due to recirculation as the ground source pump stays on). The heat pump provides heat for DHW preparation during the day in 3 periods of around 1h each and works discontinuously at night modulating the gas power to provide hot water for space heating purposes at around 3 C according to the heating curve. For a winter day (Figure 4 right) the heat pump is in operation for almost the whole measured period and the gas burner continously modulated. The gas burner modulation is a consequence of the system control: the heat pump changes frequently between DHW production and space heating mode, and so the outlet set temperature (T HW,out,SET ) between 7 and 6-55 C. The gas burner is modulated down when the set temperature decreases and it is even stopped if the hot water outlet temperature is above the new set point. This combined modulation and start/stop operation mode is seen more in detailed in figure 5, presenting heat pump operation for a day in May. For the first 7 operation hours T HW,out,SET values between C are calculated from the heating curve, for ambient temperatures falling from 14 to 1 C. In this period the inlet hot water temperature is just 3 to 1K below this level and even with the minimum burner modulation level the outlet temperature rises too far away from the set point, and start/stop operation starts. The heat pump is steadily working in start/stop mode if the difference between set point and hot water temperature return is smaller than around 5K. At around 7:3 the set temperature falls below the inlet temperature and the heat pump stops producing hot water. One hour later there is a DHW demand (T CT2 <6 C) and the heat pump resumes operation at full load. T GS,IN T GS,OUT T set 6 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213

7 Temperature in C 6 T B2 T B1 T SH2 T HW,out,set Solution pump status T HW,out T HW,in 2: 4: 6: 8: 15/Mai/212 Figure 5: HP temperature profiles for a day in May power modulation Some other aspects of the system influence the heat pump performance. The continuous change between DHW and space heating mode shown in Figure 4 right is a consequence of T CT2 flipping between 6 and 61 C within just a few minutes, being T CT1 below 6 C for all this time. The system control assumes that hot water for DHW has to be supplied but most of the water at the upper part of the combined tank seems to be at an adequate temperature level. Thus the control system forces the heat pump to a continous heat input modulation. It has also been observed that for certain operation periods (as in figure 5), T B1 is far above T HW,out,SET with T B2 being almost as high as T B1 but the heat pump runs in start/stop operation to satisfy space heating demand. A better control strategy should stop the heat pump for such periods and also stop both hot water and ground source pumps if no heat is produced (figure 5 circa 8h). 6. SUGGESTIONS FOR PERFORMANCE IMPROVEMENT The conditions defining when heat supply for DHW production is needed given the temperatures at the upper part of the combined tank (T CT2 and T CT2 ) must be changed. Heat pump operation in space heating mode should be avoided if the temperature at the lower part of the buffer (T B2 ) is almost as high as the set temperature (T HW,out,SET ) as shown in figure 5. Thus the condition to stop charging the buffer tank must also be redefined. The 25 C limit for space heating operation should be changed to a lower value to avoid night summer operation as shown in figure 4 left. Another modification to avoid part load and start/stop in space heating mode is presented in figure 6 right: the set value from the heating curve is compared with a minimum T set,min in order to ensure heat pump operation at a not-too-low part load. Only if the set value from the heating curve is higher than this minimum, T set,curve will be passed to the heat pump control. Otherwise T set,min will be used as T HW,out,SET. T ambient T HW,IN Q MI N = (Minimal part load) Q NOM Tset,DHW (7 C) MAX Tset,HP AHP DDC Figure 6: alternative hydraulic set up (left) and suggested control strategy (right) AHP Heating curve T SET,curve T SET,heating Burner modulation T hw,out,set,min = Q MIN/m c p + T HW,IN MAX T SET,MIN NEW PART 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213 7

8 System hydraulics contributes as well to heat pump start/stop operation. Water from the space heating circuit (T SH2 ) flows into the buffer which contains water at a higher temperature (figure 5).The temperature at the heat pump inlet is then higher than that that would be present if T SH2 would not flow into the buffer tank. Figure 6 left presents an alternative hydraulic set-up to avoid it, being buffer and lower part of the combined tank in series for charging cycles. The influence of the solar collector field must be carefully analysed to ensure that the heat supplied by the collectors does not affect the heat pump operation, and its control must be optimised. 7. CONCLUSIONS The monitored heating system operates reliably with seasonal gas utilization efficiency values between 1.14 and However when comparing the performance figures of the heat pump with manufacturer data it becomes clear than there is still much room for improvement. The heat pump has been shown to be working often in an inefficient start/stop mode and modulating the burner capacity. To avoid this to happen an alternative set-up is suggested together with a modified control strategy that raises the hot water outlet temperature and should bring the heat pump out of inefficient part load and start/stop working conditions. In addition to that the system control conditions for DHW and space heating demand must be changed to avoid unnecessary operation. The suggested actions are planned to be tested in this site, in the laboratory and in simulations. ACKNOWLEDGMENTS This project is supported by the European Union and the federal state of Berlin within the Environmental Relief Programme (Umweltentlastungsprogramm II, contract No. 1132UEPII/2). NOMENCLATURE AEF Auxiliary energy factor Subscripts AHP Absorption heat pump atm atmospheric n norm conditions DHW Domestic hot water avg average b buffer tank E Electrical energy [kwh] CT combined tank ref reference conditions GUE Gas utilization efficency GS ground source seas seasonal H Si Higher heating value [MJ/Nm 3 ] HW hot water SH space heating H i Lower heating value [MJ/Nm 3 ] m measured sys system Q Heat [kwh] V Gas volume [m 3 ] REFERENCES 1. Ainardi, E, Guerra, M. 28, GAHP: the most efficient heating technology. Proceedings of the International Sorption Heat Pump Conference, Seoul, Corea 2. ISO, 1995, Guide to the Expression of Uncertainty in Measurement, International Organization for Standardisation, Geneva. 3. Moser H, Rieberer R. 211, Analysis of a gas-driven absorption heat pump system used for heating and domestic hot water preparation, 1 th IEA Heat Pump Conference. 4. Nitchske-Kowsky P, Weißing W. 211, Gas Heat Pumps in Europe. Emerging Gas Technology integrating Renewable Energy. International Gas Union Research Conference. 5. Robur S.p.A (June 212), certificate_e3a-e3gs pdf 8 5th IIR Conference: Ammonia Refrigeration Technology, Ohrid, 213