sustainability
Article Impact of Compressor Drive System Efficiency on Air Source Heat Pump Performance for Heating Hot Water
Mariusz Szreder 1,* and Marek Miara 2 1 Faculty of Civil Engineering, Mechanics and Petrochemistry, Warsaw University of Technology, 09-400 Plock, Poland 2 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg, Germany; [email protected] * Correspondence: [email protected]; Tel.: +48-24-367-22-74
Received: 7 November 2020; Accepted: 14 December 2020; Published: 16 December 2020
Abstract: A standard Polish household with a central heating system powered by a solid fuel furnace was chosen as a case study. The modular Air Source Heat Pump (ASHP) was used to heat the hot water outside the heating season. In this article comparative studies of the impact of the compressor drive system used on the energy efficiency of the heat pump have been carried out in operating conditions. The ASHP heating capacity and coefficient of performance (COP) were determined for the outside air temperature in the range from 7 to 22 ◦C by heating the water in the tank to a temperature above 50 ◦C. For the case of a fixed speed compressor, average heating capacity in the range 2.7 3.1 kW and COP values in the range 3.2 4.6 depending on the evaporator supply air − − temperature were obtained. Similarly, for the inverter compressor, the average heating capacity in the range of 2.7 5.1 kW was obtained for the frequency in the range of 30–90 Hz and COP in the range − 4.2 5.7, respectively. On cool days, the average heating capacity of the heat pump decreases by 12%. − For the simultaneous operation of two compressors with comparable heating capacity, lower COP values were obtained by 20%.
Keywords: energy efficiency; inverter compressor; COP; air source heat pump
1. Introduction
The problems of global warming and the CO2 emission reductions observed in recent years have become popular topics of research around the world. Over the recent years, there has been a significant growth in the global demand for energy consumed by central heating systems in buildings [1–3]. Therefore, the activities of the European Community are focused on increasing the use of Renewable Energy Sources (RES) to reduce the demand for primary energy. The latest recommendations of EU directives [4,5] indicate heat pumps as basic devices for future generations of heating and cooling systems. This is due to the nature of heat pumps that use renewable energy from natural heat sources. The ability to use rich resources such as geothermal, aerothermal, and solar energy makes heat pumps environmentally friendly [6–8]. In comparison with traditional systems based on gas boilers and chillers, the use of heating systems using heat pumps allows the reduction in the intake of primary energy for cooling and heating systems within the buildings [9,10]. In the long-term, both climate and energy policies and decarbonization plans for EU economies until 2050 will have the greatest impact on increasing the use of heat pumps. The share of heat pumps in heating buildings and heating systems in 2050 will be crucial in each of the analyzed decarbonization options. Technology that allows the integration of energy production and consumption sectors is the most effective. Not only does it ensure the transfer of over 70% of heat as RES but also enables the effective decarbonization of heat in buildings thanks to the combination of energy production
Sustainability 2020, 12, 10521; doi:10.3390/su122410521 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 10521 2 of 17 and consumption sectors [11]. According to the European Commission, the mainstream of financial support in the new EU budget perspective in 2021 2027 will be directed at the energy efficiency of − new and modernized buildings. The report of the Polish Organization of Heat Pump Technology PORT PC asserts that the number of heat pumps sold in Poland is systematically growing, especially in the air heat pump segment [12]. In the first half of 2019, the market for air-to-water heat pumps used for central heating systems doubled compared to the sales in the first half of 2018. The market for ground heat pumps recorded an increase of 20% in the first half of 2019. During that time, the entire heat pump market in Poland recorded an increase of 30%. The increase in sales is especially visible in the segment of devices with heating capacity below 20 kW which results from the introduction of subsidies under the Clean Air Program and increased customer interest in the zero-emission heating technology. It is estimated that at the current rate of market development, the share of heat pumps in new single-family buildings will equal to 25% [13,14]. Nevertheless, over 30% of households are still heated with coal-fired boilers. In the case of such installations, it is particularly advisable to use air source heat pumps for Domestic Hot Water (DHW) heating. It is possible to exclude coal-fired boilers from hot water production in the summer and limit the emission of substances harmful to the natural environment. The widespread use of solar collectors for the DHW production [15,16] and photovoltaic panels for the production of electricity [17,18], especially in single-family housing, is the effect of introducing co-financing into renewable energy subsidy programs under RES. A photovoltaic installation is the most effective when the building’s demand for electricity is large and the electricity produced serves for the owner’s private use. Using the electric energy to power up the heat pump enables heating the building in winter and cooling it in summer, and hot water production throughout the year [19,20]. Research on the use of thermal energy from solar collectors to power up the heat pump’s evaporator circuit is also being carried out on a large scale. According to studies, a water heater with an Air Source Heat Pump (ASHP) source obtains a higher coefficient of performance (COP) efficiency factor than the SAHP system at night only [21,22]. The available test results present that the largest exergy losses appear in the compressors of heat pump system compressors [23,24]. A series of tests of new design solutions for compressor drive systems using inverter devices were carried out [25–27]. The effect of variable compressor speed on the heating capacity of the heat pump and COP was studied. Compared to classic solutions, significant energy savings of up to 20% have been achieved. Currently, the number of implementations of hybrid systems [28–30] and cascade solutions [31,32] is increasing. Grossi et al. [33] conducted energy efficiency tests of a Dual-Source Heat Pump system (DSHP) capable of using heat energy from air and soil. Simulation tests show that it is possible to reduce investment costs by reducing the DSHP well area by 15–50% compared to a conventional solution. Research on energy optimization through the introduction of hybrid systems and smart energy networks the smart grid is currently being carried out. Optimally managed communication between − − all participants of the energy market is aimed at reducing costs and integrating distributed energy sources, including renewable energy [34–36]. The available literature lacks the results of verification studies of the energy efficiency of heat pumps in real operating conditions. The aim of this article is a comparative analysis of the impact of the compressor drive system used on the energy efficiency of a typical heat pump offered in retail sales. The test results of a commercial heat pump under real operating conditions will help answer the question of whether there is sufficient economic justification for the need to use inverter drives in air source heat pumps for heating hot water.
2. Materials and Methods
2.1. Hot Water Preparation by Heat Pumps While designing an installation for the DHW production cooperating with a subsystem of a heat pump, it is necessary to begin with providing the adequate comfort of use, therefore larger storage tanks are used. During the work of the ASHP, the storage tank should take in the amount of energy that Sustainability 2020, 12, 10521 3 of 17 the heat pump currently supplies. When designing the system, it must be remembered that the power which the water tank can pick up is to be at least equal to the heat pump’s power. For heat pumps up to 10 kW, the water tank with a coil can be integrated with the heat pump housing. The lower part of the tank has a heat exchanger in the form of a coil with a larger surface (the minimum area is 0.2–0.25 m2/kW heat pump power in DHW mode). Another possible solution is the use of double-shell tanks with a capacity of 0.3–0.5 m3, which can be combined with a heat pump with a capacity of up to 15 kW. A buffer tank-type tank in the tank can be used for higher heat pumps. Then the external tank with the capacity of 0.3–0.8 m3 is connected to the heat pump. This type of tank can be also equipped with a coil to cooperate with additional heat sources, e.g., solar collectors. Having accurate information on the type of building, its purpose, and the profile of hot water consumption is crucial during the process of the installation cooperating with the heat pump.
2.2. Experimental Configuration A test stand has been appropriately designed and constructed for conducting operational tests regarding the energy analysis of a heat pump for DHW production. The basic component of the built test stand is a commercial heat pump offered by Hewalex. The monoblock heat pump, which is dedicated to installation in existing heating systems, has been used as an additional device. The possibility of connecting external water storage tanks of various capacities to the heat pump is an advantage of this solution. The designed ASHP subsystem is a device designed to work in an on/off mode. The basic data of the elements of the test stand equipment are given in Table1. According to the manufacturer’s data provided in the catalog card, the ASHP reaches a rated heating power of 2.5 kW (profile A7/W30-35). In order to conduct comparative tests of the influence of the applied compressor drive system on the energy efficiency of the heat pump, the ASHP subsystem was additionally equipped with an inverter compressor DA110S1C-30FZ.
Table 1. List of test stand elements.
Equipment Specification Value Panasonic 5PS102EAA Cooling capacity 2410 W Fixed speed compressor Input power 850 W Displacement 10.2 cc GMCC DA110S1C-30FZ Cooling capacity (for 60 Hz) 3200 W Variable speed compressor Input power (for 60 Hz) 850 W Displacement 10.9 cc Motor BLDC 12–120 Hz Tube in shell Heat Exchanger GAH03-CMF Condenser Heating capacity 7 kW Flow rate 1.2 m3/h Lamella type evaporator model 80600156 Evaporator Cooling capacity 5.5 kW Air flow rate 380 m3/h Storage tank Single shell tanks 2 0.13 m3 ×
This solution enables comparative efficiency tests of two compressors with fixed and variable speed under the same real operating conditions. Besides which, the same test method and measuring equipment used at the test stand eliminate the discrepancies in the measurement results to a minimum. Figure1 gives the basic characteristics of both compressors for the R410A refrigerant. The catalog data of the DA110S1C-30FZ compressor show that the allowable range of compressor operation is 30–90 Hz. The measuring series for this compressor was carried out in real conditions with 15 Hz steps. A dedicated Ruking iD826s inverter is offered by the manufacturer to drive this compressor. SustainabilitySustainability 2020 2020, 12, 12, x, FORx FOR PEER PEER REVIEW REVIEW 4 of4 of17 17 Sustainability 2020, 12, 10521 4 of 17 30–9030–90 Hz. Hz. The The measuring measuring series series for for this this compressor compressor was was carried carried out out in in real real conditions conditions with with 15 15 Hz Hz steps.steps. A A dedicated dedicated Ruking Ruking iD826s iD826s inverter inverter is isoffere offered dby by the the manufacturer manufacturer to to drive drive this this compressor. compressor. TheTheThe Modbus ModbusModbus RTU RTURTU communication communicationcommunication protocol protocolprotocol is is isused usedused for forfor communicationcommunication communication between betweenbetween the thethe measuring measuringmeasuring and andand controlcontrolcontrol system systemsystem and andand the thethe inverter. inverter.inverter. The TheThe connection connectionconnection di diagramdiagramagram of ofof individual individualindividual co componentscomponentsmponents of ofof the thethe ASHP ASHPASHP subsystemsubsystemsubsystem is is ispresented presented in in Figure Figure Figure 2. 2 2.The. The The collection collection collection of of ofheat heat heat energy energy energy from from from the the theheat heat heat pump pump pump is ispossible ispossible possible for for twofortwo water two water water tanks tanks tanks (0.13 (0.13 (0.13m m3 each)3 meach)3 each) independently. independently. independently. In In this this In way, thisway, it way, itwas was it possible was possible possible to to carry carry to carryout out tests tests out of tests of a heata ofheat a pumpheatpump pumpwith with a with avariable variable a variable heat heat heatload. load. load. A A standard Astandard standard 60 60 W 60 W circulation W circulation circulation pump pump pump was was was applied applied applied for for the thethe water waterwater circulationcirculationcirculation at atat the thethe heat heatheat pump pumppump outlet. outlet.outlet. Circulation CirculationCirculation pu pumppumpmp settings settingssettings made mademade it it itpossible possiblepossible to toto regulate regulateregulate the thethe water waterwater flowflowflow rate raterate in inin the thethe range rangerange of ofof 0.17–0.27 0.17–0.270.17–0.27 kg/s.kg kg/s./s.
Figure 1. FigureFigure 1. 1.Catalogue CatalogueCatalogue data datadata for forfor fixed fixedfixed and andand variable variablevariable speed speedspeed compressor. compressor.compressor.
Figure 2. FigureFigure 2. 2.Functional FunctionalFunctional diagram diagramdiagram of ofof heat heatheat pump pumppump for forfor Domestic DomesticDomestic Hot HotHot Water WaterWater (DHW).(DHW). (DHW).
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2.3. Measured Parameters and Data Acquisition The extended, commercial Hewalex heat pump for DHW heating has been tested in real use conditions. Additional sensors for measuring important parameters which are necessary to perform energy analysis of thermodynamic processes in the heat pump were installed in the test stand. The test stand has been equipped with a specialized measurement and control system for controlling the heat pump and recording the measurement data. To analyze the electricity consumption of each component during operation or to calculate COP values, electricity power meters were installed for all devices. Besides which, a second independent COP coefficient measuring system offered by the heat pump manufacturer as an additional device was installed in the test stand. In this way, a second independent measuring path was obtained with a data recording step at every 2 min. The measurement data are recorded every 30 s in the basic measurement and control system. The set measurement frequency allows the determination of the instantaneous heating power and demand for electricity. Class A wattmeters (1% accuracy) were used to record electricity consumption. The heat energy produced by the ASHP is determined by measuring the mass flow rate of water at the condenser and measuring the inlet and outlet water temperature. The mass flow rate of water was measured by the Siemens MAG 1100F electromagnetic flow meter. Special PT100 Class A RTD sensors were used to measure water temperature. They are respectively paired and specially scaled. The registration of the heat flux is carried out with a measurement uncertainty below 1%. Basic parameters of the measurement and control system are given in Table2.
Table 2. Basic parameters of measuring equipment.
Transducer Range Accuracy Electromagnetic flowmeter 0–1.5 m3/h 0.2% ± Resistance temperature sensor 50–+150 C 0.1 C − ◦ ± ◦ K-type thermocouples 100–+400 C 1 C − ◦ ± ◦ Low pressure sensor 0.1–+1.2 MPa 0.3% − ± High pressure sensor 0.1–+3.4 MPa 0.3% − ± Active power transducer 1000 imp/kWh 0.5% ±
K-type thermocouples mounted on pipes with clamps and additionally thermally protected were used to measure temperatures in the freon cycle. The condensation and evaporation pressure measurements were carried out for R410A using pressure transducers AKS32R. The registration of the above parameters is necessary to perform an energy analysis in the ASHP subsystem. In this test, the heating capacity Q was calculated based on the Equation (1)
Q = Cp,w Mw (Twout T ) (1) × × − win Input power demand W was calculated based on the Equation (2)
W = Wcp + Waux (2)
Input power demand Wmt of the Brushless DC motor was calculated by Equation (3)
Wmt = η Wcp (3) inv ×
The cooling capacity Qc was calculated by Equation (4)
Qc = Q Wcp (4) − Sustainability 2020, 12, 10521 6 of 17
The COP of the ASHP subsystem at any time (t) was calculated by Equation (5)
Q (t) COP = (5) W (t) Sustainability 2020, 12, x FOR PEER REVIEW 6 of 17 The average values of COP in each cycle was calculated by Equation (6)
R τ ( ) 𝑄Q𝑡(t𝑑𝑡)dt 𝐶𝑂𝑃 == 0 (6) COPmc R τ𝑊(𝑡)𝑑𝑡 (6) W(t)dt 0 The tested heat pump in the monoblock version is designed for central heating installations with The tested heat pump in the monoblock version is designed for central heating installations with solid fuel boilers. In this way, DHW production outside the heating season is carried out without the solid fuel boilers. In this way, DHW production outside the heating season is carried out without the need to operate coal boilers which are harmful to the environment. Therefore, operational tests of the need to operate coal boilers which are harmful to the environment. Therefore, operational tests of the commercial heat pump were conducted for positive air temperatures supplied to the ASHP commercial heat pump were conducted for positive air temperatures supplied to the ASHP subsystem. subsystem. 3. Experimental Results 3. Experimental Results The heat pump’s measuring cycles were carried out for outside air temperatures of 7, 15, 22 ◦C. The heat pump’s measuring cycles were carried out for outside air temperatures of 7, 15, 22 °C. During the measuring cycles, the water in the storage tank was heated from room temperature to During the measuring cycles, the water in the storage tank was heated from room temperature to the the temperature above 50 ◦C. The measurement cycles were carried out for three cases of compressor temperature above 50 °C. The measurement cycles were carried out for three cases of compressor operation: operation: factory fitted typical compressor, •• factory fitted typical compressor, additional inverter compressor, •• additional inverter compressor, • simultaneous operation of two compressors. • simultaneous operation of two compressors.
3.1.3.1. Heat Heat Pump Pump with with Fixed Fixed Speed Speed Compressor Compressor InIn this this variant, variant, the the tests tests on on the the commercial commercial heat heat pump pump were were conducted conducted under under real real operating operating conditions.conditions. Figure Figure 3 presents recorded changes in air temperature in the evaporator circuit and water temperaturetemperature in in the the condenser condenser circuit circuit for for profile profile A22/W30-50. A22/W30-50.
FigureFigure 3. 3. GraphGraph of of registered registered temperatures temperatures for for measurement measurement cycle. cycle.
On average, 20 min after the start of the test cycle, the operating conditions in the refrigerant circuit stabilize. The difference in water temperature in the condenser was ∆Tw = 3.2 °C on average. In the presented example, a stable increase in evaporation temperature from 7 °C to 11 °C can be observed. At the start of the measurement, the air temperature difference in the evaporator circuit was ∆Ta = 16 °C, and at the end of the measurement was ∆Ta = 11 °C. During the measuring cycle, 6.92 kWh of heat energy was produced, 1.94 kWh of electricity was consumed, and COP = 3.61. For the
Sustainability 2020, 12, x FOR PEER REVIEW 7 of 17 Sustainability 2020, 12, 10521 7 of 17 determined thermodynamic balance of the heat pump, the evaporation temperature is comparable to the air temperature at the outlet of the evaporator. On average, 20 min after the start of the test cycle, the operating conditions in the refrigerant Instantaneous COP and heating capacity values were determined every 30 s for each circuit stabilize. The difference in water temperature in the condenser was ∆T = 3.2 C on average. measurement cycle. Recorded data are shown in Figure 4. The compressor inputw power◦ increased In the presented example, a stable increase in evaporation temperature from 7 C to 11 C can be from 660 W to 1050 W in proportion to the increase in condensing temperature. ◦ ◦ observed. At the start of the measurement, the air temperature difference in the evaporator circuit In the whole measuring cycle, an average heating capacity of 3100 W was obtained. The was ∆T = 16 C, and at the end of the measurement was ∆T = 11 C. During the measuring cycle, instantaneousa ◦COP value for the compressor changed froma 5.35 ◦to 3.75. The additional devices 6.92 kWh of heat energy was produced, 1.94 kWh of electricity was consumed, and COP = 3.61. For the (circulation pump, fan) consumed an average input power of 120 W. Finally, the average COP value determined thermodynamic balance of the heat pump, the evaporation temperature is comparable to for the measuring cycle dropped to 3.68. The heating capacity remained constant during the measurementthe air temperature cycle, atwhile the outlet the demand of the evaporator. for the compressor input power increased with increasing condensingInstantaneous temperature. COP and Therefore, heating capacitythe demand values for werecooling determined power decreases, every 30 swhich for each results measurement from the Equationcycle. Recorded (4). This data explains are shown the increase in Figure in4 .evaporation The compressor temperature input power and the increased evaporator from 660outlet W air to temperature.1050 W in proportion to the increase in condensing temperature.
Figure 4. Graph of registered heating power, coefficient coefficient of performance (COP) and input power for measurement cycle.
FigureIn the whole 5 presents measuring the averaged cycle, an values average of heating heatin capacityg power of and 3100 COP W was for obtained.three values The of instantaneous the outside airCOP temperature. value for the As compressor expected, changed the heat from pump’s 5.35 to op 3.75.erating The additionalefficiency devicesincreases (circulation as the outside pump, fan)air temperatureconsumed an increases. average inputOn cool power days, of the 120 heat W. pump Finally,’s theenergy average efficiency COP decreases value for theby 12%. measuring The impact cycle ofdropped the water to 3.68. flow The rate heating on the capacity energy remainedefficiency constant of the tested during heat the pump measurement was described cycle, while in the the previous demand author’sfor the compressor studies [37]. input power increased with increasing condensing temperature. Therefore, the demand for cooling power decreases, which results from the Equation (4). This explains the increase in evaporation temperature and the evaporator outlet air temperature. Figure5 presents the averaged values of heating power and COP for three values of the outside air temperature. As expected, the heat pump’s operating efficiency increases as the outside air temperature increases. On cool days, the heat pump’s energy efficiency decreases by 12%. The impact of the water flow rate on the energy efficiency of the tested heat pump was described in the previous author’s studies [37].
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Figure 5. 5. CalculatedCalculated heating heating capacity, capacity, COP COP and andinputt input powerpower power forfor measurementmeasurement for measurement cyclecycle forfor cycle classicclassic for compressor.classic compressor. 3.2. Heat Pump with Variable Speed Compressor 3.2. Heat Pump with Variable Speed Compressor An additional variable-speed compressor was installed on the test stand to test the impact of the An additional variable-speed compressor was installedled onon thethe testtest standstand toto testtest thethe impactimpact ofof thethe compressor used on the heat pump’s efficiency. Measurement cycles were carried out in the same way compressor used on the heat pump’s’s efficiency.efficiency. MeasurementMeasurement cyclescycles werewere carriedcarried outout inin thethe samesame as in the item above. Figure6 compares the COP value and heating capacity of the inverter compressor way as in the item above. Figure 6 compares the COP value and heating capacity of the inverter for profile A22/W30-50. The heat pump allows you to adjust the heating capacity in the range from compressor for profile A22/W30-50. The heat pump allows you to adjust the heating capacity in the 2700 W for 30 Hz to 5100 W for 90 Hz. The highest COP was obtained for the frequency of 30 Hz. range from 2700 W for 30 Hz to 5100 W for 90 Hz. The highest COP was obtained for the frequency The COP value decreased proportionally to the increase in condensing temperature in the range from of 30 Hz. The COP value decreased proportionally to thethe increaseincrease inin condensingcondensing temperaturetemperature inin thethe 5.70 to 4.20 (falls by 25%). The lowest COP values were recorded for 90 Hz. range from 5.70 to 4.20 (falls by 25%). The lowest COP valuesvalues werewere recordedrecorded forfor 9090 Hz.Hz.
FigureFigure 6. 6. CalculatedCalculated heating heating capacity, capacity, COP for measurement cycle for inverter compressor.
The impact of the outside temperature on on the the effici efficiencyency of of the the heat heat pump pump is is shown shown in in Figure Figure 77.. At aa frequencyfrequency of of 60 60 Hz, Hz, the the heat heat pump pump has has a heating a heating capacity capacity of 3900 of 3900 W for W profile for profile A22/ W30-50.A22/W30-50. A drop A drop in outside temperature by every 7 °C forces a decrease in heating capacity by 6%. For the
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Sustainability 2020, 12, x FOR PEER REVIEW 9 of 17 Sustainabilityin outside 2020 temperature, 12, x FOR PEER by every REVIEW 7 ◦ C forces a decrease in heating capacity by 6%. For the A22/W30-509 of 17 profile, the heat pump generates an average of 5.55 kWh of heat energy, consuming 1.55 kWh of electric A22/W30-50 profile, the heat pump generates an average of 5.55 kWh of heat energy, consuming 1.55 A22/W30-50energy with profile, an average the heat COP pump= 3.70. generates an average of 5.55 kWh of heat energy, consuming 1.55 kWhkWh of of electric electric energy energy with with an an average average COP COP = = 3.70. 3.70.
FigureFigure 7. 7. 7. Calculated CalculatedCalculated heating heating heating capacity, capacity, capacity, COP COP COPand and input andinput inputpowe power powerrfor for variable variable for variable air air temperature temperature air temperature and and inverter inverter and compressor.invertercompressor. compressor.
FigureFigure 88 8 shows showsshows the the BLDC BLDC BLDC motor’s motor’s motor’s demand demand demand for for for input input input power power power needed needed needed to to drive drive to drive the the compressor. thecompressor. compressor. The The compressorThecompressor compressor input input power input power powerdemand demand demand increases increases increases in in proportion proportion in proportion to to the the increase increase to the in in increasefrequency frequency in and and frequency condensing condensing and temperature.condensingtemperature. temperature. The The measurement measurement The measurement results results obtained obtained results are are obtained co consistentnsistent are with consistentwith the the catalog catalog with thedata data catalog given given datain in Figure Figure given 1in1 for for Figure a a given given1 for evaporation evaporation a given evaporation temperature. temperature. temperature.
FigureFigure 8. 8. Change Change of of input input power power demand demand by by Brushless Brushless DC DC motor. motor.
TheThe Ruking Ruking inverter inverter dedicated dedicated by by the the compresso compressor rsupplier supplier was was used used to to drive drive the the BLDC BLDC motor. motor. FromFrom previous previous measurements measurements carrie carriedd out out by by the the author author a a relatively relatively low low inverter inverter efficiency efficiency of of 80% 80%
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The Ruking inverter dedicated by the compressor supplier was used to drive the BLDC motor. Sustainability 2020, 12, x FOR PEER REVIEW 10 of 17 From previous measurements carried out by the author a relatively low inverter efficiency of 80% was wasnoted noted [38]. [38]. Therefore, Therefore, comparative comparative tests weretests alsowere carried also carried out using out the using universal the universal Invertek OptidriveInvertek OptidriveE3 inverter. E3 Comparativeinverter. Comparative results are results given are in Figuregiven 9in. FromFigure the 9. measurementsFrom the measurements obtained itobtained appears itthat appears the Optidrive that the Optidrive inverter worked inverter with work aned average with an e ffiaverageciency efficiency of 90%. of 90%.
FigureFigure 9. 9. ChangeChange of of input input power power demand demand by by the the inverter. inverter.
3.3.3.3. Heat Heat Pump Pump Version Version with with Two Two Comp Compressorsressors with with Fixed Fixed and and Variable Variable Speed Speed TheThe expansion expansion of of the the commercial commercial heat heat pump pump wi withth aa second second compressor compressor allows allows testing testing of of the the devicedevice with with two two compressors compressors running running simultaneously. simultaneously. Therefore, Therefore, it it is is possible possible to to perform perform additional additional teststests of of the the effect effect of of oversizing oversizing of ofcompressor compressor po powerwer on onthe the energy energy efficiency efficiency of the of theheat heat pump. pump. A chartA chart of recorded of recorded temperature temperature changes changes for the for A22/W30-50 the A22/W30-50 profile profile for two for measuring two measuring cycles cycles with 0.13 with 3 3 m0.133 and m 0.26and m 0.263 water m tankswater is tanks given is in given Figure in 10. Figure For a10 si.ngle For water a single tank, water a high tank,er dynamics a higher dynamicsof water temperatureof water temperature increase in increase the condenser in the condenser circuit was circuit noted was (increase noted (increaseof 30%). The of 30%). diagram The diagramon the left on showsthe left a showssituation a situationwhen a variable when a speed variable compressor speed compressor was intentionally was intentionally started 10 started min after 10 minthe start after ofthe the start measuring of the measuring cycle. It can cycle. be observed It can be that observed the moment that theof switching moment ofon switching the second on compressor the second introducescompressor a introducesmomentary a disturbance momentary to disturbance the thermodynamic to the thermodynamic balance of the balance refrigerant of the refrigerantcircuit. A significantcircuit. A significantdecrease in decrease evaporation in evaporation temperature temperature is also visible. is also Besides visible. which, Besides the which, difference the diff inerence the dynamicsin the dynamics of the ofcondenser the condenser water water temperature temperature increase increase for the for thecase case of switching of switching on on one one and and two two compressorscompressors can can be be observed. observed. TheThe comparison comparison presentedpresented inin Figure Figure 11 11shows shows that that the heatthe heat pump pump has obtained has obtained a heating a heating capacity 3 3 capacityof 5000 Wof for5000 stable W for operating stable operating conditions conditions for a 0.13 m forstorage a 0.13 tank m3 storage and about tank 4000 and W about for 0.26 4000 m storageW for 0.26tanks. m3 Thestorage graph tanks. on theThe right graph shows on the that right for shows the same that profile for the A22 same/W30-50, profile the A22/W30-50, heat pump achievedthe heat pumpthermodynamic achieved thermodynamic equilibrium at a lowerequilibrium evaporation at a lower temperature evaporation (decrease temperature of 4 ◦C). The(decrease greater of demand 4 °C). Thefor coolinggreater powerdemand causes for cooling a decrease power in thecauses air temperature a decrease in in the evaporatorair temperature circuit in and the inevaporator addition a circuitdecrease and in in evaporation addition a temperature.decrease in evaporation As a result, thetemperature. graph on theAs righta result, shows the agraph decrease on inthe heating right showspower a bydecrease 1000 W in and heating a decrease power in by the 1000 average W and COP a decrease value by in 0.15. the average COP value by 0.15.
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FigureFigure 10.10.10. Graph GraphGraph of ofof registered registeredregistered temperatures temperaturestemperatures for measurementforfor measurmeasurementement cycle cycle incycle the in Airin thethe Source AirAir HeatSourceSource Pump HeatHeat (ASHP) PumpPump. (ASHP).(ASHP).
FigureFigure 11. 11.11.Graph GraphGraph of registeredofof registeredregistered heating heatingheating capacity, capacity,capacity, COP and COPCOP input andand power inputinput for power operationpower forfor of operationoperation two compressors. ofof twotwo compressors.compressors. Figure 12 represents the heating power and COP when the inverter compressor frequency is changingFigureFigure in 12 the12 representsrepresents range 30, 45,thethe 60 heatingheating Hz. For powerpower the second andand COPCOP and whenwhen third case,thethe inverterinverter the ASHP compressorcompressor obtained frequency afrequency maximum isis heatingchangingchanging power inin thethe of rangerange 5000 W. 30,30, A 45,45, fixed 6060 Hz. speedHz. ForFor compressor thethe secondsecond and andand an thirdthird inverter case,case, compressor thethe ASHPASHP forobtainedobtained 60 Hz aindividuallya maximummaximum obtainheatingheating heating powerpower power ofof 50005000 above W.W. 3000AA fixedfixed W. In speedspeed the case comprecompre of thessorssor simultaneous andand anan inverterinverter operation compressorcompressor of two compressors, forfor 6060 HzHz theindividuallyindividually maximum obtainobtain heating heatingheating power powerpower of the aboveabove ASHP 30003000 is limited W.W. InIn the bythe the casecase water ofof thethe flow simultaneoussimultaneous rate and the operationoperation heating powerofof twotwo ofcompressors,compressors, the condenser thethe (Table maximummaximum1). This heatingheating means powerpower that the ofof heatingthethe ASHPASHP capacity isis limitedlimited of the byby condenser thethe waterwater has flowflow been raterate correctly andand thethe selectedheatingheating powerpower in the designofof thethe condensercondenser (appropriate (Table(Table oversizing). 1).1). ThisThis meansmeans The highest thatthat thethe energy heatingheating effi capacitycapacityciency was ofof thethe obtained condensercondenser for hasthehas secondbeenbeen correctlycorrectly case, average selectedselected values: inin thethe 4900 designdesign W heating (appropriat(appropriat capacity,ee oversizing).oversizing). 1650 W input TheThe power, highesthighest COP energyenergy= 3.0 efficiency wereefficiency obtained. waswas obtainedobtained forfor thethe secondsecond case,case, averageaverage values:values: 49004900 WW heatingheating capacity,capacity, 16501650 WW inputinput power,power, COPCOP == 3.03.0 were were obtained. obtained.
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Sustainability 2020, 12, 10521 12 of 17 Sustainability 2020, 12, x FOR PEER REVIEW 12 of 17
Figure 12. Calculated heating capacity, COP and input power for fixed and variable speed compressors. FigureFigure 12.12. CalculatedCalculated heating heating capacity, capacity, COP COP and inputand powerinput forpower fixed andfor variablefixed and speed variable compressors. speed 3.4. Energy and Economic Analysis compressors. 3.4. Energy and Economic Analysis The summary of the measurement results presented in the points above shows that a heat pump 3.4.with Energy Thea fixed summary and speed Economic of compressor the Analysis measurement can provide results heating presented capacity in the pointsin the above range shows of 2200 that W a heatto 3100 pump W withdependingThe a fixed summary speedon the compressor ofoutside the measurement air can temperature provide results heating with presented capacity an average in in the the pointsCOP range valueabove of 2200 inshows W the to 3100 thatrange Wa heat dependingof 3.2–4.6. pump withSimilarly,on the a outsidefixed a heat speed air pump temperature compressor with a variable with can anprovide speed average compressorheating COP valuecapacity can in generate thein rangethe rangeheating of 3.2–4.6. of capacity 2200 Similarly, W in to the 3100 arange heat W dependingpumpof 2700 with W atoon variable 5150the outsideW, speed depending compressorair temperature on current can generate with setting an heating saverage and current capacity COP operating invalue the rangein theconditions. of range 2700 W of Figure to 3.2–4.6. 5150 13 W, Similarly,showsdepending a comparisona onheat current pump settingsof with a heat a andvariable pump current speedoperating operating compressor parameters conditions. can generatefor Figure the 13 A22/W30-50heating shows acapacity comparison profile in thefor of range athree heat ofpumpcompressor 2700 operating W to configuration 5150 parameters W, depending variants. for the on A22 current/W30-50 setting profiles and for threecurrent compressor operating configuration conditions. Figure variants. 13 shows a comparison of a heat pump operating parameters for the A22/W30-50 profile for three compressor configuration variants.
Figure 13. AverageAverage values of heating capacity, COP andand input power demand for measurement cycle (profile(profile A22A22/W30-50)./W30-50). Figure 13. Average values of heating capacity, COP and input power demand for measurement cycle The diagram shows that for a variable-capacity compressor, a heat pump can provide a heating (profile A22/W30-50). capacity in the range of 2700 W to 5100 W at frequencies in the range of 30–90 Hz. The input power increases on average by 38% in proportion to the increase in frequency and the heating capacity by increasesThe diagram on average shows by 38%that forin proportion a variable-capacity to the increase compressor, in frequency a heat pump and the can heating provide capacity a heating by 22%. The COP value, on the other hand, varies from 4.8 to 3.1 (a decrease of 35%). In the case of capacity in the range of 2700 W to 5100 W at frequencies in the range of 30–90 Hz. The input power increases on average by 38% in proportion to the increase in frequency and the heating capacity by
Sustainability 2020, 12, x FOR PEER REVIEW 13 of 17 Sustainability 2020, 12, 10521 13 of 17 22%. The COP value, on the other hand, varies from 4.8 to 3.1 (a decrease of 35%). In the case of hybrid operation, higher heating capacity values were obtained but with a higher demand for input power, whichhybrid resulted operation, in higherlower COP heating values. capacity The values most werebeneficial obtained effects but were with aobtained higherdemand for the forinverter input compressorpower, which in resultedthe low infrequency lower COP range, values. but heating The most the beneficial water in e fftheects storage were obtained tank is burdened for the inverter with extendedcompressor operating in the lowtime. frequency range, but heating the water in the storage tank is burdened with extendedFigure operating 14 presents time. a summary of the daily electricity consumption for DHW production in a householdFigure for 14 selected presents heat a summary pump operation of the daily varian electricityts. As the consumption compressor drive for DHW frequency production increases, in a thehousehold electricity for consumption selected heat pumpincreases operation linearly. variants. Consequently, As the compressor the unit cost drive of frequencyDHW preparation increases, increases.the electricity The consumption lowest operating increases costs linearly. are obtained Consequently, at 30 theHz unitbut costthey of result DHW preparationin the heat increases.pump’s extendedThe lowest operating operating time. costs For are the obtained 45 Hz at frequency 30 Hz but of they the result inverter in the compressor, heat pump’s the extended heating operatingcapacity wastime. comparable For the 45 to Hz that frequency of the fixed of the speed inverter compressor, compressor, but the a 10% heating lower capacity daily wascost comparableof DHW was to achieved.that of the The fixed impact speed of compressor, varying daily but DHW a 10% demand lower daily can cost be reduced of DHW by was using achieved. storage The tanks impact with of increasedvarying daily water DHW capacity demand and, can if po bessible, reduced starting by using the storageheat pump tanks in with the low-frequency increased water range. capacity In and,the householdsif possible, startingheated with the heatsolid pump fuel boilers, in the low-frequencynot only does the range. production In the households of DHW by heated means with of a heat solid pumpfuel boilers, outside not the only winter does season the production offer an economic of DHW byand means maintenance-free of a heat pump solution outside but the also winter a positive season effectoffer anon economicthe environment. and maintenance-free solution but also a positive effect on the environment.
FigureFigure 14. 14. DailyDaily costs costs of of DHW DHW prepar preparationation in in a a household. household.
4.4. Discussion Discussion TheThe overall overall energy energy efficiency efficiency of of a a heat heat pump pump unde underr real real operating operating conditions conditions is is influenced influenced by by manymany parameters parameters characterizing characterizing the the operating operating conditions conditions of of individual individual heat heat pump pump components components [39]. [39]. IfIf the the heat heat pump’s pump’s components components have have been been chosen chosen correctly, correctly, the the choice choice of of a a compressor compressor and and drive drive methodmethod has has a a decisive decisive impact impact on on its its overall overall efficiency. efficiency. The The use use of of a a fixed fixed speed speed compressor compressor allows allows goodgood operating operating parameters parameters to to be be achieved achieved only only for foroptimal optimal conditions conditions of use. ofuse. As expected, As expected, in the in case the ofcase a variable of a variable speed speed compressor, compressor, an increase an increase in he inating heating capacity capacity was was obtained obtained in in proportion proportion to to the the increaseincrease in in compressor compressor frequency frequency and and a a decrease decrease in in COP COP with with an an increase increase in in condensing condensing temperature. temperature. TheThe obtained obtained researchresearch resultsresults are are consistent consistent with with the the research research results results presented presented by other by other research research teams. teams.Wang etWang al. [40 et] al. conducted [40] conducted tests of tests the inverter of the inve heatrter pump heat in pump the frequency in the frequency range from range 50 Hz from to 11050 Hz Hz toin 110 10 HzHz stepsin 10 andHz steps with and a constant with a condensingconstant condensing temperature temperature set at 45, set 55, at 65 45, and 55, 75 65◦C. and The 75 available °C. The availableresearch resultsresearch from results constant from speedconstant compressor speed compressor driven heat driven pumps heat and pumps variable and speed variable compressor speed compressorheat pumps heat show pumps that an show inverter-driven that an inverter-driven heat pump is heat an e pumpffective is way an effective to improve way the to annual improve energy the annualsaving energy due to saving better partialdue to loadbetter effi partialciency. load Cuevas efficiency. et al. [Cuevas41] reported et al. that[41] reported the losses that of thethe electriclosses ofmotor the electric attributable motor to attributable the use of theto the inverter use of arethe insignificant, inverter are insignificant, as the tested as inverters the tested worked inverters with workedefficiency with in theefficiency range of in 95–98%. the range Karlsson of 95–98%. and Fahlen Karlsson [42] provedand Fahlen that there[42] proved is no improvement that there is in no the
Sustainability 2020, 12, 10521 14 of 17 annual energy efficiency of an inverter-driven heat pump compared to a conventional fixed speed heat pump, mainly due to the inefficient operation of the inverter and compressor motor. Chae and Ren [43] carried out tests of a heat pump equipped with two compressors with fixed and variable speed. They showed that a fixed speed compressor can run more efficiently under constant load conditions, meeting 50% of the heat demand. In the event of increased instantaneous heat demand, the inverter compressor is switched on at part load. The results obtained in this article show that compared to a single inverter compressor operation at 90 Hz, hybrid operation (50 Hz fixed speed + 30 Hz inverter) produced a comparable amount of heat but with a 15% lower COP.
5. Conclusions The energy efficiency of an air source heat pump with fixed and variable speed compressors for hot water production was examined in this study. The obtained experimental results confirm that the heating capacity of the tested heat pump is sufficient to cover the demand for the thermal energy needed to prepare domestic hot water. The heat pump performance remained constant during each measurement cycle for stable operating conditions. The heating capacity of the heat pump with a fixed speed compressor varied from 2700 W for an outside air temperature of 7 ◦C to 3200 W for an air temperature of 22 ◦C. COP values changed in the range of COP = 3.2–4.6, respectively. For the heat pump variant with a variable speed compressor in proportion to the 30–90 Hz frequency range, the heat pump heating capacity varied from 2700 W to 5100 W, and the COP value was in the 3.1–4.80 range, respectively. The refrigerant’s condensing temperature has a significant impact on the instantaneous COP value. On cool days, the average heating capacity of the heat pump decreases by 12%. A variable heat pump ensures flexible adaptation of the heating capacity to the varying hot water demand. If possible, it is recommended to prepare DHW in the lower frequency range which results in the lowest operating costs. However, the extended working time is needed to perform this task. For heating DHW, it is most advantageous to use an oversized, variable speed compressor operating in the lower frequency range.
Author Contributions: Conceptualization, M.S., M.M.; Methodology, M.S., M.M.; Investigation, M.S.; Data curation, M.S.; Writing—original draft preparation, M.S.; Writing—review and editing, M.S., M.M.; Visualization, M.S.; Supervision, M.S. All authors have read and agreed to the published version of the manuscript. Funding: The APC was partially funded by a grant Warsaw University of Technology to support research activities in the discipline of mechanical engineering (grant No. 504/04499/7193/43.090002). Conflicts of Interest: The authors declare no conflict of interest.
Nomenclature
COP coefficient of performance, [-] Cp specific heat, [kJ/kg K] M mass flow, [kg/s] Q heating or cooling capacity, [W] T temperature, [◦C] W input power, [W] Greek Symbols η efficiency, [-] τ time, [s] Abbreviations A22/W30-50 Air at 22 ◦C/Water in the upper source at 30–50 ◦C A7/W30-35 Air at 7 ◦C/Water in the upper source at 30–35 ◦C ASHP air source heat pump Sustainability 2020, 12, 10521 15 of 17
DHW domestic hot water DSHP dual source heat pump RES renewable energy sources Subscripts a air aux auxiliaries c cooling con condenser cp compressor ev evaporator fix fixed speed in inlet inv inverter mc measurement cycle mt motor out outlet w water
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