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Compressor Selection and Equipment Sizing for Cold Climate Heat Pumps

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Compressor Selection and Equipment Sizing for Cold Climate Heat Pumps Poster P.6.11 - 1 - Compressor Selection and Equipment Sizing for Cold Climate Heat Pumps Bo Shen, Omar Abdelaziz, C. Keith Rice Building Equipment Research Group, Building Technologies Research and Integration Center, Oak Ridge National Lab, TN, USA Abstract: An extensive array of air source heat pump design and sizing options were investigated to identify solutions to limit heating capacity degradation at -13°F (-25°C) ambient to ≤25%, compared to the nominal rating point capacity at 47°F (8.3°C). Sixteen equipment design options were evaluated in one commercial building and one residential building, respectively in seven cities. The energy simulation results were compared to three baseline cases: 100% electric resistance heating, a 9.6 Btu/Wh (2.8 W/W) heating seasonal performance factor (HSPF) single-speed air-source heat pump unit, and 90% AFUE gas heating system. The general recommendation is that variable-speed compressors and tandem compressors, sized such that their rated heating capacity at a low speed matches the building design cooling load, are able to achieve the capacity goal at low ambient temperatures by over-speeding. For example, in a home with a 3.0 ton (10.6 kW) design cooling load, a tandem heat pump could meet this cooling load running a single compressor, while running both compressors to meet heating load at low ambient temperatures in a cold climate. Energy savings and electric resistance heat reductions vary with building types, energy codes and climate zones. Oversizing a heat pump can result in larger energy saving in a less energy efficient building and colder regions due to reducing electric resistance heating. However, in a more energy-efficient building or for buildings in warmer climates, one has to consider balance between reduction of resistance heat and addition of cyclic loss. Key Words: Heat Pump, HVAC Equipment Modeling, Building Energy Simulation, Heat Seasonal Performance Factor 1 INTRODUCTION In the U. S., there are approximately 14.4 million dwellings that use electricity for heating in very cold and cold regions with an annual energy consumption of 0.16 quads (0.17 EJ). A high performance air-source cold climate heat pump (CCHP) would result in significant savings over current technologies (> 70% compared to strip heating). It can result in an annual primary energy savings of 0.1 Quads (0.1055 EJ) when fully deployed, which is equivalent to 5.9 million tons (5.35 million MT) of annual CO 2 emissions reduction. For cold climate heat pumps, the primary market segment consists of existing and new residential buildings in cold climate regions using electricity as primary heating source. Electric resistance furnaces are used in cold climates with limited access to natural gas to provide heating as the result of the severe capacity loss and energy performance degradation experienced by conventional heat pumps in extreme ambient conditions. Resistance heating is limited to a maximum COP of 1.0. Cold climate heat pumps have the capability to provide much higher energy efficiency, i.e. significant energy savings. Presently, current technical and economic barriers limit market penetration of heat pumps in cold climates. R&D efforts can be employed to overcome these barriers and develop high performance CCHPs that minimize, or even eliminate, the need for backup strip heating. Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. 11th IEA Heat Pump Conference 2014, May 12-16 2014, Montréal (Québec) Canada Poster P.6.11 - 2 - The major R&D challenges are to limit the heat pump heating capacity degradation and enhance the operation efficiency as much as possible at extremely low ambient temperatures. The heating capacity of a regular heat pump using a single-speed compressor decreases drastically as the ambient temperature drops, while the building heating demand increases. Consequently, a regular heat pump can’t match the heating demand at extremely low ambient temperatures, and significant supplemental resistance heat has to be used, which decreases the heat pump annual operation efficiency. In addition, Bertsch et al. (2005) pointed out that due to the high pressure ratio and low suction pressure at low ambient temperatures the compressor discharge temperature could exceed maximum compressor operating limits thus limiting the working range of the heat pump. It is difficult to size a CCHP properly - if the rated heating capacity matches house load at low ambient temperatures excessive cycling and efficiency losses occur at moderate ambient temperatures. Researchers have investigated several cycle configurations for CCHP. Bertsch et al. (2006, 2008) studied two-stage compression with three alternatives, i.e. with oil cooling between the low-stage and high stage compressors; with inter-stage economizing; with inter- stage heat exchanger (cascade cycle). These cycle configurations effectively lower the discharge temperature and maintain a good efficiency at low ambient temperatures. However, those options have not been implemented successfully on the market, due to the oil return difficulty and much higher initial cost. Therefore, our focus is on “conventional” technologies, which can be implemented on a large scale, with reasonable cost increment, to investigate the potential of single-stage compression using available compressors on the market. Our analyses are based on equipment system modeling and building energy simulation. Heat Pump Design Model (HPDM) is a public-domain, hard-ware-based, heat pump design and modeling tool, developed by Rice et al. (2005). In order to model compressors, HPDM uses AHRI 10-coefficient compressor maps to calculate mass flow rate, power consumption; simulate energy balance from inlet to outlet using the calculated power and given heat loss ratio; it also considers the actual suction state to correct the map mass flow prediction. For heat exchanger modeling, It uses a segment-to-segment modeling approach; Each tube segment has individual air side and refrigerant side entering states, and considers possible phase transition; An ε -NTU approach is used for heat transfer calculations within each segment. Air-side fin is simplified as an equivalent annular fin. Both refrigerant and air-side heat transfer and pressure drop are considered; the coil model can simulate arbitrary tube and fin geometries and circuitries, any refrigerant side entering and exit states, misdistribution, and accept two-dimensional air side temperature, humidity and velocity local inputs. It is capable of modeling both condenser and evaporator. EnergyPlus 7.2 (2013) is used for the building energy simulations discussed in this paper. EnergyPlus uses off-design performance curves to correct heat pump capacities and power consumptions from the design condition at one speed level (i.e. Cooling: 80°F(26.7°C) indoor dry bulb/67°F(19.4°C) indoor wet bulb, 95°F (35°C) outdoor dry bulb temperature; Heating: at the outdoor temperatures of 47°F (8.3°C) DB/ 43°F (6.1°C) WB and indoor temperature of 70°F (21.1°C)), to account for impacts of off-design temperatures. The curves are normalized to the heat pump rating point performance. To facillitate variable speed modeling capability, EnergyPlus 7.2 requires inputting normalized performance curves at individial speed levels, and linearly interpolates performance between two neighboring speed levels. The real-time operating speed is chosen by the model, matching the coil capacity to the building load. Our analyses are based on compressor maps provided by a manufacturer, and calibrated system and heat exchanger models of HPDM. HPDM is used to generate off- design performance curves by running steady-state simulations in an extensive working range. The performance curves are used in EnergyPlus for building energy simulations. 2 Heat Pump Equipment Modeling The base air source heat pump unit we selected for the analyses is a 5-ton, split heat pump, using a single-speed, 5-ton (17.6 kW) scroll compressor, having rated SEER 11th IEA Heat Pump Conference 2014, May 12-16 2014, Montréal (Québec) Canada Poster P.6.11 - 3 - (seasonal energy efficiency ratio) of 13.0 Btu/WH (cooling seasonal performance factor, CSPF, of 3.8 W/W) and HSPF of 9.6 Btu/Wh (2.8 W/W). The unit information, i.e. heat exchangers, lines, fans, etc. was provided by our industry partner. We used HPDM to model the equipment and calibrated the model against the manufacturer’s published performance data. After this was done, we kept the same heat exchangers, indoor blower and outdoor fan, and refrigerant connection lines, and evaluated different compressor options including: a 2- stage 5-ton scroll compressor (the top speed provides 5-ton cooling capacity at the rated condition, and ratio of capacity between the low and high stage is 67%/100%); a 2-stage 6- ton scroll compressor (67%/100% capacity ratio); a tandem 8-ton scroll compressor (consisting of two identical 4-ton compressors); a variable speed (VS) scroll compressor (having 5-ton nominal cooling capacity at 4500 RPM); a larger size variable speed scroll compressor (20% more mass flow rate and power consumption than the 5-ton VS at each speed); a smaller size, 5-ton tandem scroll compressor (consisting of two identical 2.5-ton compressors); and a 5-ton single-speed vapor injection (VI) scroll compressor. It should be noted that the variable speed compressors investigated have speed ranges of 1800 RPM to 7200 RPM. The refrigerant charge of a typical heat pump unit is normally optimized in cooling mode.
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