ENERGY MODELLING OF A GAS-FIRED ABSORPTION HEAT PUMP IN A RESIDENTIAL HOME FOR COLD CLIMATE USING TRNSYS

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1 ENERGY MODELLING OF A GAS-FIRED ABSORPTION HEAT PUMP IN A RESIDENTIAL HOME FOR COLD CLIMATE USING TRNSYS KAJEN ETHIRVEERASINGHAM (MASc Student) Department of Mechanical and Industrial Engineering Ryerson University 350 Victoria St. Toronto, Ontario, Canada kethirve@ryerson.ca ALTAMASH A. BAIG (PhD Candidate) Department of Mechanical and Industrial Engineering Ryerson University 350 Victoria St. Toronto, Ontario, Canada altamashahmad.baig@ryerson.ca ALAN S. FUNG (PhD, PEng, FCSME) Department of Mechanical and Industrial Engineering Ryerson University 350 Victoria St. Toronto, Ontario, Canada alanfung@ryerson.ca ABOUT THE AUTHOR Kajen Ethirveerasingham, is currently a MASc Student in Mechanical Engineering at Ryerson University. His research interests are energy modelling, energy management, experimental validation, high performance buildings and Net-Zero Energy Homes (NZEH) and HVAC equipment. Kajen has previously worked on experimental validation and modelling of Air-Source Heat Pump Water Heaters (ASHPWH) at test faculties in the Archetype Sustainable Houses located in Vaughan, Ontario, Canada. Currently he is the lead TRNSYS modeler for a Toronto Solar home project, known as EcoStudio, a collaborative effort with Ryerson University, University of Toronto and Seneca College. ABSTRACT In Canada, cold winters result in significant energy usage for space and water heating in residential homes, resulting in higher energy consumption and expenses to the home owners. Most of the numerous sustainable technologies avaliable are powered by electricity, and with rising electricity prices in Canada, natural gas is still seen as the most cost-effective option. To keep operation costs low, while also reducing the emissions, transitional technologies are a potential option to avoid using costly electricity as the primary source. Gas-fired Absorption Heat Pumps (GAHP) for residential use is a recent concept for heating and cooling purposes. Manufacturer performance data was used to develop a TRNSYS 17 model for both the heating and cooling season. This model is used to describe the GAHP providing space heating and cooling for an Archetype house located in Vaughan, Ontario, Canada. The results of the simulation showed that the GAHP had a higher efficiency than that of a typical furnace with a Coefficient of Performance (COP) of 1.08 during the 1

2 heating season, but showed poor performance during the cooling season with a COP of However, compared to the operational costs of an Air Source Heat Pump (ASHP) system, the GAHP was found to have a reduced annual cost of $ Short cycling was an issue that was shown to be significant through the simulation with 3.1 times and 2 times per hour during the heating and cooling season respectively with only 5-minute cycle durations. The performance of the GAHP will be experimentally evaluated in a test facility at Archetype Sustainable Houses in Vaughan where the data will be used to validate manufacturer performance data and develop a more effective control strategy to reduce short cycling. 1. INTRODUCTION Gas-fired Absorption Heat Pumps (GAHPs) conceivably have the potential to deliver space conditioning and water heating at a higher level of efficiency and lower operating cost than traditional gas heating equipment. In comparison with electric heat pumps, GAHP technology has many potential merits, such as more stable thermal outputs in cold weather conditions, higher primary energy conversion efficiency, reduce peak electricity demand and lower operating costs. GAHPs present a potentially viable transitional technology since their operation costs are lower than electric heat pumps and their emissions are lower relative to traditional gas heating equipment. A recent examination of residential heat pump systems in Ontario by the Independent Electricity Systems Operator (IESO) has found that electric heat pump technologies will benefit homes that already use electricity as the primary heating source, but are less beneficial for homeowners that use natural gas as the primary heating fuel [1]. They have also taken into consideration that gas prices would need to increase by more than 50% for electric heat pumps such as Air-Source Heat Pumps (ASHP) to be viable alternatives in this climatic context [1]. What the examination report did not consider however, is that transitional gas driven technologies such as GAHPs can offer a middle ground, in terms of costs and emission reduction, between both ASHP and conventional heating systems such as furnaces and boilers. In Canada, the most common heating appliance used, in 57% of Canadian households, is the forced air furnace [2]. Furnaces have an upper limit in efficiency of 100%, however, a GAHP system can utilise energy beyond the limit imposed on furnaces, making it a potentially viable upgrade. 1.1 Objective A GAHP system will be modelled and experimentally validated through in-depth field performance monitoring and analysis. This will be done at the Toronto Regional Conservation Authority s (TRCA) Sustainable Archetype Houses located in Vaughan, Ontario, Canada. At this location, there are two attached Archetype houses, House A and House B. Currently, modeling and experimental work will be done for House A, which can be seen in Figure 1. The primary research objective is to analyse the greenhouse gas (GHG) emissions reductions, potential energy and cost savings along with identifying efficient control strategies. This will be done through modelling of the GAHP in the Archetype Houses using TRNSYS 17. This paper presents the results of modelling the GAHP unit to provide annual space heating and cooling demand of the Archetype House A. Understanding the performance of the GAHP will help further improve how effectively it can be implemented for residential applications, where cost to the homeowners is a driving factor. 2

3 Temperature ( C) Figure 1. Attached Sustainable Archetype Houses (House B on far left, House A on right). 2.1 House A model 2. MODEL INFORMATION The weather data that was used for this simulation was for the metro Toronto area and had a minimum and maximum temperature of C and C respectively. The external ambient temperature profile can be found in Figure 2. The model was developed so that only heating will be provided during the heating season and only cooling for the cooling season Hour (hr) Figure 2. Annual external temperature profile for metro Toronto Area. The house model used was developed and validated for simulation and experimental validation of an Air Source Heat Pump (ASHP) in previous research performed at the same location [3]. The House A model used has an approximate volume of 810m 3 and a floor space of approximately 335m 2 (3600ft 2 ). House A is designed to use a forced air HVAC system as opposed to House B which uses a hydronic HVAC system. The total annual heating and cooling demand for House A can be seen in Figure 3. The peak heating and cooling demand for this house was found to be 6.92kW and 4.71kW respectively. The GAHP unit however, is oversized for this demand as it provides 35kW for heating and 17kW for cooling and was the most avaliable unit for both heating and cooling in the region. A dump coil, where excess energy can be dumped, is available at the test 3

4 GAHP Output (MJ/hr) Thermal Demand (kw) facility that is expected to be able to dump 65% of the total energy of the GAHP. Due to this, the GAHP was sized to 35% of its capacity for this model. This results in a GAHP heating capacity of 12.25kW and a cooling capacity of 5.95kW Hour (hr) Heating Demand (kw) Cooling Demand (kw) 2.2 GAHP model Figure 3. Space heating and cooling demand for House A. The model developed in this paper is designed to present a base case of use of the GAHP system with water as the primary coil fluid at nominal flow settings according to the manufacturer. This will be validated and compared to cases with different primary fluids and mechanical system designs. For heating, the flow rate is set to 13.4 GPM and during cooling the flow rate is 12.8 GPM [4]. When the GAHP operates, the nominal power consumption is 0.75kW [4]. The manufacturer provided performance data to describe the output of energy of the GAHP depending on the external temperature which is accepted as valid. This data was curve fitted into polynomial functions that had R 2 values approaching unity. Plots of the correlation and the resulting equations and R 2 values can be seen in Figure 4 and 5. These functions were evaluated for nominal operating settings of the GAHP. These equations were input into TRNSYS using a simple control strategy to operate the GAHP y = 1E-06x 5-4E-06x x x x R² = External Ambient Temperature ( C) Figure 4. GAHP performance curve for heating for GAHP inlet fluid temperature of 40 C [4]. 4

5 GAHP Output (MJ/hr) y = -8E-05x x x x R² = Temperature ( C) Figure 5. GAHP performance curve for cooling for GAHP inlet fluid temperature of 12 C [4]. 2.3 Control strategy During heating season, the fluid is to be heated from 40 C to 50 C and during cooling season, the fluid is cooled from 12 C to 7 C. Only one setting was used as values cannot be interpolated between different fluid set points due to the complicated nature of the absorption process. This fixed set point of heating the inlet fluid from 40 C and cooling the inlet fluid from 12 C is used to develop a control strategy to describe the GAHP operating procedures. These were the nominal set points as given by the manufacturer for the GAHP unit. For the implementation of the GAHP system, the two main components are the GAHP unit itself and the pump for the hydronic loop of the GAHP. The pump has a nominal operating power of 375 W and can satisfy flow rates for both heating and cooling season [5]. The GAHP is programmed to only activate once the primary fluid in the loops drops either below 40 C for the heating season or is above 12 C for the cooling season. Since the GAHP system is oversized, heat is expected to remain in the hydronic loop which will still need to be pumped to meet demand. For this reason, the control of the pump is set to operate when there is demand in the house, yet the GAHP control is set to operate only when both the demand of the house and the primary fluid temperature conditions are met. If there is unmet demand in the house but return temperature is too high, the pump will continue to work. Once the fluid temperature reaches the set point, the GAHP will activate until the return temperature is once again above or below the threshold, depending on the season. Due to this strategy, a large time step such as 1 hour is not feasible as the returning fluid may be heated far above or be cooled far below the GAHP set points, introducing error that cannot be easily accounted for. The simulation was performed with 5-minute time step intervals to better mimic real operation. 3. RESULTS AND DISCUSSION A summary table of the results obtained from the simulation that characterise the efficiency and cost effectiveness of the GAHP can be found in Table 1. As seen from the total demand during the heating and cooling season, the control was sufficient to meet the demands. The Gas Utilisation Efficiency (GUE) coefficient shows how effective the GAHP is with respect to gas consumption alone. For the heating and cooling season, the GUE was found to be 1.16 and 0.69 respectively.the Coefficient of Performance (COP) is similar to the GUE, but takes into account electrical power consumption as well. The COP of the GAHP was evaluated by 5

6 dividing the GAHP output by the sum of the energy consumed from the natural gas and GAHP and pump electrical power. The heating and cooling COP value solved was 0.95 and 0.53 respectively, which shows a significant decrease in comparison to the GUE, which only accounts for gas consumption. However, with the inclusion of the circulation pump power as energy consumed for operation, the COP for heating and cooling season drops to 0.95 and 0.53 respectively. From the GUE value, the efficiency is above 100% in energy utilisation. This surpasses the ability of furnaces, implying that GHG emissions is reduced per unit of energy produced. One team in Montreal, Canada has done experimental testing in similar cold climates using a similarly sized GAHP unit and were able to achieve a heating and cooling COP of 1.12 and 0.55 respectively [6]. The cooling COP experimentally closely matches the simulation results, however, there is some significant deviation in heating COP. This is largely due to the addition of supporting technologies such as thermal storage and geothermal heat exchangers improving the experimental testing results. This helps to highlight the importance of supporting technologies to improve the system to be more feasible. Season Heating Cooling Thermal Demand (kwh) GAHP Operating (Fired) Hours GAHP Output (kwh) Gas Consumption (kwh) Gas Consumption (m 3 ) Pump Power (kwh) GAHP Power (kwh) GUE COP Table 1. Summary of GAHP operation in House A for heating and cooling season. The results of the simulation for GAHP output for both heating and cooling season can be seen in Figure 6. In this figure, it can be seen that there are many cycles that the GAHP goes through. On average, it was found that the GAHP cycles 3.1 times an hour during heating season and 2 times anhour during cooling season. Previous research work on GAHP systems has suggested that the GAHP must operate for 30 minutes at minimum to achieve steady state operating conditions [7, 8]. However, from the results of the simulation, the GAHP would only be fired for 5 minute durations at minimum and 10 minutes at maximum. These values are also greatly influenced by the time step of the simulation. From these results, thermal inertia and part load operation implementation into the model should be investigated. The results obtained from experimental work may show that manufacturer given performance data is more optimistic than would be seen in real world operation, since it uses only state state operating data, for the current system design and house demand. 6

7 GAHP Output (kw) Hour (hr) GAHP Heating Output (kw) GAHP Cooling Output (kw) Figure 6. Annual GAHP energy output for House A. 2.3 Cost analysis One of the main selling points of the GAHP is that it operates mainly on natural gas, which is much more affordable than electricity in many regions, mainly in Vaughan, Ontario where this system will be field tested. Understanding the operational cost savings compared to an ASHP will offer key insights into feasibility. ASHPs are very efficient units that have a far greater COP values than current GAHP systems. The ASHP system that was performance tested in the Archetype Sustainable House had shown to achieve COP values of 3.32 and 5.27 for heating and cooling season respectively [9]. Previous modelling work on the ASHP has shown that during the heating and cooling seasons, the ASHP consumes 5442kWh and 434kWh of electricity seasonally to meet energy demands of House A for a total consumption of 5876kWh annually excluding pumps and fans [9]. The total electricity that the GAHP consumes is 1404kWh annually and 1784m 3 of natural gas (at 37 MJ/m 3 [2]). The average marginal unit costs (which includes transportation and distribution costs) for electricity and gas are evaluated, using current prices in the Toronto, Ontario region, to be /kwh and /m 3 [10, 11]. HVAC Unit ASHP GAHP Total Natural Gas (m 3 ) Total Electricity (kwh) Total Cost ($) Difference ($) Table 2. Summary of GAHP operation in House A for heating and cooling season. Table 2 presents a breakdown of energy consumption and cost comparison between the ASHP and GAHP. The GAHP has significantly lower operating costs due the savings gained using natural gas as the primary fuel. Excluding the power consumption due to fans and pumps, an annual operating costs savings of $ can be obtained from using a GAHP over an ASHP if correctly sized. 4. VALIDATION AND EXPERIMENTATION Validation of the model will be done through field testing the GAHP unit in House A located in Vaughan, Ontario. The monitoring system implemented to measure all crucial variables such as 7

8 flow rate and temperature in the primary loops along with outdoor relative humidity and temperature. All sensors are calibrated, where temperature sensors have uncertainty of <1% and flow sensors to have an uncertainty <2%. The Archetype houses have been used for field testing various HVAC equipment as a result, as a plethora of sensors installed throughout the home for a complete and detailed monitoring plan. Cooling season testing is expected to begin July 2017 where the data will be used to develop steady state performance curves and optimize control strategies. The dump coil will also be used to meet the full capacity of the GAHP to help reduce cycling and part load performance issues. Performance data collected during this period will also be used to validate manufacturer data. When field testing, various strategies such as using a 50% propylene glycol/water mixture and thermal storage tanks will be used to identify where further improvements can be made to increase the performance of the system. CONCLUSIONS During the simulation, two main issues were identified: short cycling and accounting for thermal inertia and part load performance. The GAHP has showed consistently that cycle times of 5 minutes would only occur 2-3 times an hour on average during the year. This is due to the oversizing of the heat pump where methods to reduce the significance must be investigated. Increasing the demand of the home and implementing thermal storage systems are both promising options that may help to maintain longer cycles. The 5-minute operating time however, can be attributed mainly to the time step as a lower time step may result in lower operating time. Since the performance data used is steady state based, low operating times cannot be adequately represented without the use of a more complex model that will take into consideration the behavior of the absorption heat pump for such short durations. As expected, the GAHP showed higher efficiency during the heating season compared to more common heating systems such as forced air furnaces with a COP of 1.08 without a pump, whereas a furnace is limited to 100% efficiency. The performance during the cooling season however is exceptionally low, at a COP of 0.64 without a pump. Having a low COP during the cooling season means inefficient use of natural gas, thus larger GHG emissions. Whether or not carbon savings from the heating season can be transferred over to the cooling season to compensate for the reduced efficiency and remain competitive with other HVAC systems such as an ASHP, should be investigated. Relative to operational costs however, the GAHP shows promise compared to the ASHP, at least with respect to electricity and natural gas prices in the Toronto, Ontario region. The GAHP showed to have an annual operating cost of $658.96, where the ASHP had a cost of $823.35, resulting in a cost reduction of $ annually when using a GAHP system. ACKNOWLEDGEMENTS We would like to thank the staff at the TRCA Sustainable Archetype Houses, Ryerson University and its partnerships with Union Gas Limited and Enbridge Gas Distribution for providing the resources and funding for the project. We would also like to thank the NSERC CRD program, OCE TalenEdge, and MITACS for additional funding and support for this research project 8

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