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issue: July 2008 APPLIANCE Magazine

Heat Pumps
Designing Air-Source Heat Pumps for Cold Climates


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by James Bryant, mechanical engineer, Hallowell International

Boosted compression technology is the basis for a new heat pump designed specifically for colder climates.

Hallowell International used boosted compression technology in the Acadia series combined heating and cooling systems.

Air-source heat pumps are the dominant heating source in the southern United States and in many places around the globe. They typically present minimum vapor cycle efficiencies in excess of 200% and offer savings in operating expenses over fossil fuel and conventional direct electric heating systems. Heat pumps have traditionally been ignored in cold climates because their performance envelopes are limited to an ideal range, and falling outside of those bounds on the heating side leads to high operating costs and discomfort in the home.

In the southern parts of the United States, air-source heat pumps are known for low operating costs and reliability. Moving equipment north abates these virtues, and fossil fuels become more prevalent in heating spaces. This is unfortunate for consumers. According to the U.S. Department of Energy’s Energy Information Administration (EIA), over the past decade, fossil fuel prices have increased an average of about 12% annually. Electricity, on the other hand, has been more stable, increasing less than 3% annually on the average for the same time period (see Figure 1). This makes electricity an inviting source of heating energy.

Heat pumps in northern climates suffer a number of limitations stemming from the fact that they are designed for air-conditioning applications. As climates become cooler and heating becomes more of the primary HVAC function, one may find that conventional heat pumps lose capacity and do not satisfy the load of the conditioned space. In colder temperatures, a conventional system’s need to defrost can further detract from heating performance. The use of resistance heat or fossil fuels to supplement or replace the vapor cycle often makes these systems expensive to operate. The additional use of supplemental heat to temper cold air blowing into the space during defrosts further contributes to high operating costs. As it gets colder outside, the delivery air temperatures inside begin to fall when no supplemental heat is being used. Supply air temperatures that are warmer than the return temperatures add heat to a space, but discomfort occurs when these supply temperatures drop below skin temperature. The air movement can feel cool or even cold to the consumer. While the heat pump may be operating exactly as intended, and very efficiently at that, the consumer will desire a more comfortable environment.

High electric use, discomfort, and operating expenses are not only undesirable for the consumer, but for electric utilities as well. As utilities constantly seek ways to balance loads, reduce seasonal peaking, and generate more revenue to pursue more-efficient means of generation, a conflict is created when a conventional system relies on resistance heat. The high electric usage in the heating season drives up winter demand and can make heat pumps an unattractive technology in a northern climate.

Electric utilities are looking for technologies to compete with fossil fuels. They are presented with yet another conflict when directly marketing and supporting fossil fuels in dual-fuel applications. Fossil fuels are an energy source they might rather compete with directly. Until recently, geothermal heat pumps have been the only realistic option. Geothermal heat pumps have their own drawbacks, namely, high costs of installation and complications associated with less-than-perfect installations. With the costs of fossil fuels on the rise, there has never been a more opportune time to reevaluate the importance of designing an air-source heat pump for performance in extreme climates.

Figure 1. Average annual price increases: 1997 to 2007.

Applying Boosted Compression Technology

Consumers frequently ask: “How can you get heat out of air that is so cold?” To engineers, this is a simple question to answer. Heat is all around us, until we reach the absence of heat, or absolute zero (–460°F). As long as the air is warmer than absolute zero, there is heat available. The real engineering goes into how to successfully collect this heat in the vapor cycle and efficiently make it useful. This is a fundamental objective behind boosted compression technology, which is currently being used by Hallowell International in the Acadia series combined heating and cooling systems.

As temperatures fall outside, refrigeration cycle pressures and temperatures also fall. The temperatures and pressures on the low side of the system must decrease with the outside air to continue to collect heat. On the high side of the system, the same decrease in temperatures and pressures is occurring. As long as the compressor in the system is a fixed-displacement unit, this is the natural course of the vapor cycle. Eventually, a number of things will occur in these systems: Low evaporator temperatures accelerate the formation of frost on the coil, and the occurrence of defrosting completely inhibits the system’s ability to provide heating. Also, the heat content in the liquid returning from the condenser is great enough to flash the refrigerant into vapor once it travels through the metering device and into the low side of the system or evaporator. This means that the latent heat of the refrigerant is no longer being utilized, and little to no heat is collected. This results in an inefficient recycling of previously collected energy, and no practical energy is being transferred to the conditioned space.

Boosted compression addresses these issues directly. The core principle of the technology is sustaining the ability to absorb heat into the evaporator coil at extreme temperatures and making this heat practical for use in heating a space. Collecting the heat means running an ideal temperature differential on the evaporator for the range in which the system must operate. In the case of the Acadia system, the operation range is as cold as –30°F. At –30°F, a controlled –38° to –42°F evaporator is still collecting heat.

The mechanisms utilized for this process are two compressors and a liquid subcooling economizer. The two compressors are paired in series, which is unique to the existing boosted compression patents. A multispeed primary compressor performs the majority of the work, providing two stages of heating in moderate conditions and two stages of cooling for all cooling operation. When outdoor temperatures become less than ideal, the vapor cycle becomes less effective, and operation becomes uncomfortable for the owner. Boosted compression is used to bring back the ideal conditions within the system, even though its operating environment has changed. Assuming 30°F is the point at which conventional systems begin to suffer from the aforementioned problems and supplemental heat is used to maintain the temperature within the heated space, boosted compression is used to replace the inefficient supplemental heat with an efficient and effective vapor cycle. The addition of the booster compressor creates a third stage in the vapor cycle.

In boosted compression, a second, larger compressor is used in series with the primary compressor. The booster compressor continues to pull on the evaporator coil, decreasing the pressures and temperatures within the system. The load on the booster compressor remains very small as it discharges into the suction of the primary compressor. The volumetric efficiency of the booster becomes very high as the load on the discharge side is reduced by the pull of the primary compressor, and the mass flow rate throughout the system is increased, resulting in increased capacity. This yields a vapor cycle that is not only effective, but incredibly efficient: The very small load on the booster compres­sor minimizes the power consumption with respect to the primary compressor.

To help maintain operation at this temperature, a liquid subcooling economizer is implemented to further reduce the temperature of the liquid feeding the metering device. This reduces the internal energy that would preemptively flash the liquid to vapor on the low side of the system. The refrigerant is then able to absorb more energy at these conditions, helping to maintain the capacity of the system. The economizer meters a small amount of refrigerant from the liquid line and expands it, collecting heat from the warm liquid and flashing the small amount of refrigerant into a saturated vapor. This saturated vapor can be discharged into the suction stream to the primary compressor after the discharge of the booster (see Figure 2).

The system operation is staged based on sensor readings that are local to the equipment and the level of heating or cooling call from the thermostat. At any given outdoor temperature, there is a high and low capacity available from a boosted compression machine. The primary compressor, split into two equal capacities, provides two stages of operation down to 30°F, and the booster compressor and economizer provide a second stage of capacity below 30°F, where the full capacity of the primary compressor becomes the first stage.

Figure 2. Boosted compressor.

In all cases, the system may stage back from a higher stage to a lower stage to maintain steady-state operation. This allows a lower stage to continuously run where it may not satisfy the home load. When a second-stage call is generated from the thermostat, a higher capacity is entered. When the second-stage call is satisfied, the system stages back to the lower mode, and steady-state efficiencies may be maintained. This not only reduces operating costs for the owner, but also greatly increases the life of a system, as the primary compressor is not being cycled needlessly. The load on the booster compressor is so light that it never sees a hard-start condition. Couple this with the booster compressor’s typical <20% duty cycle over the life of the system, and the overall system life expectancy is greatly increased compared with a conventional system in the same environment.

Additionally, design emphasis was placed on the evaporator coil design in heating operation to reduce the need to defrost. By simply considering heating operation as a primary function and balancing coil design, quantity and mass-flow rate of refrigerant, and flow rate of the air through the outdoor coil at peak frosting conditions, a system that defrosts significantly less than a conventional system can be realized. A reduction in defrosts leads to higher efficiencies and maintained heating capacities which, in turn, reduce costs to operate in those worst-case conditions.

The resulting performance of this arrangement is a system that maintains more than 100% of its nominal heating capacity at 0°F at a 2.24 COP, and more than 80% at –15°F at a 2.06 COP. This is compared with conventional systems that retain less than 40% of nominal heating capacity at 0°F at a 2.5 COP, and less than 25% at –15°F at a 1.8 COP (see Figure 3).

Figure 3. Heating operation retained capacity.

Behind the Technology

All of this is achieved with standard off-the-shelf components found at HVAC distributors all over North America. There are no variable-speed compressors, or proverbial flux capacitors, with boosted compression. The primacy of simplicity and safety can be seen in the design and components. Bristol reciprocating compressors are used for their proven robustness over broad operating conditions. Standard Sporlan thermostatic valves (TEVs) and Parker Hannifin HVAC components are used throughout.

Some design aspects, however, are unique. A.O. Smith helped to develop a fan motor capable of operating in a broad range of conditions. The resulting “Arctic Duty” motor is more robust than a conventional HVAC motor, yet is still a standard frame-and-mount unit. The compressors are in an isolated compartment insulated with a high-quality, low E material that dampens significant amounts of noise and creates the thermal barrier necessary for minimizing heat losses from the system components. The proprietary control board hosts a multitude of safety and efficiency logic to ensure the system is always operating in the most appropriate state. All sensors are integrated into the outdoor system, so only a thermostat capable of three heating outputs and two cooling outputs is required for operation.

The Future of Heating

Boosted compression has been touted as a “green” product, although the intent was never to take that approach. In reality, the technology is as green as the local electric infrastructure allows it to be. In locales heavily dependent (more than 50% supplied) on coal for electricity, the net carbon emissions from boosted compression are comparable to those of oil and gas furnaces. However, there are many places in North America where electricity sources are renewable. Using wind power, hydroelectric, and photovoltaic technologies, there is the possibility of eliminating a carbon footprint from heating and cooling. In some cases, boosted compression systems have been paired with residential photovoltaic and wind systems, taking the owner’s power dependence almost completely off the grid.

Boosted compression is a heat pump technology set to change the status quo of heating in cold climates. Even when compared with the most recent advancements in variable refrigerant volume (VRV) technology, boosted compression offers superior performance with proven components. Boosted compression is providing solutions to problems that have been ignored for years.

About the Author

James Bryant holds a BS in mechanical engineering technology from the University of Maine at Orono, ME, U.S. His past work experience includes development and design of automated semiconductor processing equipment and boosted compression heat pump system development. If you wish to contact Bryant, e-mail lisa.bonnema@cancom.com.

 

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