Can a New Kind of Heat Pump Change the World?

An Electric-bill-burdened Engineer has developed what the HVAC industry has ignored: a heat pump that works when the temperature is below zero. Will consumers beat a path to his door?

1/6/2006 By Charles Linn, FAIA

	Platts E Source expects that the low-temperature heat pump will be competitive with conventional heat pumps and central air in Zones 1 and 2. In Zone 3, utility companies would likely have to provide consumers with incentives to get them to make the switch. Image: Courtesy Platts E Source

When David Shaw got a $400 electric bill in 1995, he was inspired. He had recently retired from his job as compressor designer and refrigeration engineer at the Carrier Corporation, and had moved into a New Britain, Connecticut, condominium that was heated and cooled by an air-source heat pump. “It worked great,” he says, “except when it got cold. The air-conditioning industry never developed a heat pump that could heat a home when it is really cold outside.” So, Shaw set up an R&D lab, Shaw Engineering Associates, and started developing the heat pump that could.

Everyone loves the idea of heat pumps, because it’s as if they give us something for nothing. Conventional air-source models heat or cool using thermal energy that is naturally present in the air, and their cousins, geothermal heat pumps, tap the heat that is present in earth or water. These devices “compress” this energy to yield temperatures required to condition interior space. Air-source types are commonly used to condition homes and small commercial buildings in the southern part of the U.S. and in many parts of the world. Yet they’ve always been very expensive to use where ambient outdoor temperatures begin to approach and go below freezing and, as the map indicates, that leaves most of the U.S. out in the cold. The reason for this is that as temperatures fall, heat pumps become less and less efficient. So, most use electric-resistance heating as a backup when a severe cold snap occurs. But that’s a bit like making buildings into giant toasters—resistance heating is not only terrible from an efficiency standpoint, but when hundreds of thousands of resistance heaters go online at the same time, electric utilities experience peak-loading. Their distribution systems are taxed, they must bring extra power plants online to meet demand, and they pay dearly to buy power from other utilities. Utility companies build these costs into their retail customers’ base rates.

The absence of viable low-temperature air-source heat-pump (LTHP) technology has left the geothermal heat pump as the only practical alternative for people who wish to use heat pumps in cold climates. The first-costs for these systems is higher than it is for fossil-fueled heaters because they are complex, and the systems that draw heat from natural sources can be difficult to install. Payback periods for them can be reasonable, but many urban and suburban sites are unsuitable because they lack either the real estate needed for ground loops or sources of water.

From a thermodynamics standpoint, the LTHP has always been possible, and Shaw says that most of the knowledge and components necessary to make LTHPs have been around “since I got in the business in 1958,” but they were never developed. Low prices for fossil fuels, and low first-costs for equipment have assured that furnaces and boilers continue to dominate the U.S. space-heating market. This didn’t deter him, and he tackled the problem in the mid-1990s, knowing full well the market forces needed to make the product a home run might not converge for years. Considering what is now known about global warming, and unprecedented prices for fossil fuels, it might be time for the LTHP to start changing the world, because it seems impossible that millions of individual residential and light-commercial heating systems, each burning its own fossil fuels, can be sustained indefinitely. In terms of carbon production alone, it is better to have hundreds of utilities produce the power to run millions of heat pumps. Electric utilities have lots of options available to them for reducing their carbon footprint that are not available to the natural gas industry. These include carbon capture and storage, nuclear power, wind generation, and other renewables.

click images to view them larger 1 2 3 The difference between a conventional heat pump (1) and the LTHP is the addition of a second “booster” compressor and a “subcooling economizer,” which is a heat exchanger. In Stage 1 mode (2), the booster compressor is activated when the outside temperature reaches 25 degrees Fahrenheit. The extra capacity it provides allows a much greater quantity of low-density refrigerant to be compressed into the liquid required to bring heat to the interior of the building. In Stage 2 (3), the economizer kicks in. It is a heat exchanger that uses heat usually wasted to produce refrigerant vapor sent directly to the primary compressor, instead of into the evaporator coil. Renderings: Courtesy Hallowell International

In 2005, Platts E Source, a Boulder-based consulting group that does research for the utility industry, released a report called “Can the Low-Temperature Heat Pump Defrost the Status Quo in the Space Heating Sector?” The authors, Jay Stein, Andria Jacob, and Jon Slowe, indicate that none of the major U.S. HVAC manufacturers is even doing research in the area of LTHPs. Without the market demand, the big companies simply aren’t interested in the concept, even though E Source estimates that the market could be as high as 2.2 million units annually.

But the paper also describes how far the LTHP has to go. Very few LTHPs of Shaw’s design—only between 150 and 200—have ever been installed. Nyle Special Products, of Bangor, Maine, licensed the rights to Shaw’s patents for a few years and made them under the Cold Climate Heat Pump name between 2002 and 2005. A number of electric utilities conducted tests of the Nyle product with mixed results, mostly due to manufacturing glitches and installation problems. When they worked, they worked very well. But Shaw decided to take his patents elsewhere, and Nyle can no longer manufacture the products that used them. Shaw has become the chief technology officer of a new company, Hallowell International (http://www.gotohallowell.com), also of Bangor. Hallowell hopes to start producing 2000 LTHPs for beta testing this year. Shaw also says that his company’s heat pump will only cost about 20 percent more than conventional heat pumps, which doesn’t seem like much, of course. But, as long as heating with natural gas or heating oil is cheaper than heating with electricity month after month, year after year, it will be hard to persuade consumers to buy them. On the other hand, utility companies often use economic incentives to push new technologies out to consumers. Those that have excess capacity to sell in winter, or experience peak-loading conditions at this time of year, are very interested in the product.

Heat Pumps 101

	The LTHP has legs (top) to keep it out of snow and ice, which improves winter efficiency. The gray cylinder is the booster compressor; the gold-colored box is the economizer (bottom).

Here’s a refresher course on heat-pump basics. Refrigerants are the life-blood of every heat pump, refrigerator, or air-conditioning system. These materials are extremely efficient at absorbing thermal energy in one place, and moving and releasing it in another. Water is often used to move thermal energy in heating and air-conditioning. But the refrigerants used in heat pumps have many advantages over water. They don’t freeze at 32 degrees Fahrenheit, and they boil at temperatures that are much lower than 212 degrees Fahrenheit. Their boiling points can also be raised or lowered significantly by pressurizing or depressurizing them, so when and where they are changed from a liquid state into a vapor or gas can be controlled. That’s very useful, because it is when they are changing states that they do their work, absorbing heat when they are changing from a liquid to a gas, and releasing it when they are changing from a gas back into liquid. Old ozone-depleting refrigerants have been replaced by new ones that are also much more efficient at absorbing and giving up heat. Today, R410A is the most commonly used refrigerant for both residential and light-commercial systems.

To understand how refrigerant works, imagine a closed bottle full of it sitting in a cold place, and assume the container is partly full of liquid refrigerant and partly of the refrigerant in a gaseous state. If it was moved to a warm place, it would gradually absorb heat from its surroundings, and as it did so, the liquid refrigerant would boil, evaporating into a gas. The pressure inside the bottle would increase until the boiling stopped, because as the pressure in the bottle increased, so would the temperature at which the liquid boils. If the bottle was put back in a cool place, the vapor would give up heat into its surroundings, condense back into liquid, and the pressure in the bottle would decrease.

The other component needed to make a heat pump from the loop of refrigerant-filled tubing is an expansion valve. This device is placed downstream from the condenser. It restricts the flow of the refrigerant inside the condenser so the compressor can build up pressure that’s necessary to condense the gas into a liquid. It also keeps this liquid from leaving the condenser before the heat it contains has fully transferred out of it. Expansion valves can be modulated, so that the amount of pressure in the condenser is variable, and the rate the condensed liquid leaves the condenser can be controlled. The pressure downstream from the expansion valve is much lower than it is in the condenser, so when warm liquid refrigerant leaves the condenser and is forced through the expansion valve, where the boiling point is also lower, some of it “flashes” into vapor. The temperature of liquid that left the condenser now becomes cold as it enters an assembly of pipes and sheet-metal fins, called the evaporator, which sits outside the building. It is at this point that any heat that remained in the warm liquid refrigerant after it left the expansion valve is boiled off into cold vapor. As it changes state, it absorbs heat from the outside air, helped along by the evaporator’s large surface area. Soon, the thermal-energy-laden vapor is on its way back to the compressor to start the cycle all over again.

What differentiates a heat pump from an air conditioner used strictly for cooling is that the direction of the refrigerant flow can be reversed—the evaporator and condenser can be switched end-for-end, so one can deliver either heat or cooling to the inside of a building. In cooling mode, the condenser is outside, and the evaporator is inside.

As the temperature starts getting near freezing outside, the amount of heat that can be absorbed by the liquid refrigerant boiling in the evaporator decreases. This is because the pressure of the boiling liquid (measured in pounds per square inch) inside the evaporator decreases, and so does the density of the vapor (measured in pounds per cubic foot) the boiling liquid turns into. This causes two problems. First, the compressor has to work harder to pump it because the pressure has dropped. Second, because the amount of heat that vapor can carry is proportional to its density, the compressor doesn’t have the capacity to deliver sufficient heat from the outside air to keep the inside of the building warm. As the temperature continues to go down, the situation worsens and, at 30 degrees Fahrenheit, the backup resistance built into most air-source heat pumps turns on.

The most obvious way to solve the problem would be to put a really big compressor into the system. But when it’s not very cold outside, this overcapacity would cause the system to be so inefficient that it would be counterproductive. So instead, Shaw decided to add a second compressor, which he calls a booster compressor (see page 164, diagram 2). This is installed between the evaporator coil and what he calls the primary compressor—the compressor that’s already present as standard equipment in every heat pump. Most of the time, the booster compressor would be bypassed, and only the primary would compress the vapor that is generated in the evaporator. When the vapor pressure and density dropped below a certain point, however, the booster compressor would be allowed to come on if the outdoor air temperature had dropped below a certain point and the thermostat inside the building is also calling for more heat. The booster compressor has a much larger displacement than the typical primary compressor, so when it is enabled, it can move many more cubic feet of vapor per minute. The LHTP’s performance can be enhanced in the future when variable speed booster compressors are introduced.

The top graph shows how well different parts of an LTHP keep up as the temperature drops. With everything running, an LTHP keeps up with heating load until 0 degrees Fahrenheit, while conventional heat pumps bottom out at 25 degrees. The graph at right shows coefficients of performance. At 0 degrees, the LTHP makes twice as much heat per unit of electricity input as the conventional heat pump.

Shaw also knew that in most heat pumps, even after the liquid refrigerant has given up much of its heat to the condenser, it is still pretty warm. When it gets really cold outside, this warmth causes as much as 40 percent of that liquid to vaporize as it goes through the expansion valve. It would be better if it cooled first. That way, more of the refrigerant would remain in liquid form, so it could be boiled later on in the evaporator coil, where it absorbs heat from the outside air while changing states. Shaw figured that one way to cool the refrigerant would be to “donate” some of its surplus heat to a process that would create a source of high-density vapor that would bypass the evaporator coil altogether and be sent directly to the primary compressor.

Shaw calls the device he uses to do this a subcooling economizer (see page 165, diagram 3). It is a heat exchanger that is placed between the condenser and the expansion valve. It splits the refrigerant liquid coming from the condenser into two streams. The majority of the refrigerant passes through one side of the heat exchanger, where it gives up the heat necessary to vaporize a smaller stream of refrigerant being fed into the other side of the exchanger. This vapor is then sent to a point between the booster compressor and the primary compressor, while the larger stream of liquid refrigerant, now cooled significantly, is sent through the expansion valve and on to the evaporator coil, where it boils into vapor.

How well LTHPs perform

The graphs on page 166 show the actual performance of the LTHP units that are now under development at Hallowell International, according to measurements taken in the company’s labs. Shaw says they have been verified by an independent lab, as well. Above 30 degrees Fahrenheit, the energy efficiency of the LTPHs is fairly consistent with most common heat pumps, but as Shaw says, “Below 30 is where the action is.” One of the graphs shows the performance of a 3-ton LTHP, in Btu per hour, compared to a conventional heat pump, as the exterior temperature falls. At 0 degrees, with the economizer, primary, and booster compressors all running, the LTHP is keeping up with the heating load, but the air-source heat pump cannot keep up below 25 degrees. The other graph shows coefficients of performance (COP) for the two heat-pump types. The COP is the ratio of the energy transferred for heating to the input electric energy used in the process—the higher the COP, the more efficiently the unit operates. Below 30 degrees, the efficiency of the heat pump using resistance heating drops very quickly. At 0 degrees, the typical air-source heat pump has basically stopped producing any heat and is using only its electrical-resistance heat, which has a COP of 1, while the COP for the LTHP is 2.23.

So, can the LTHP change the world? Not just yet. E Source’s studies show that the calculation of an owner’s payback for installing one, as compared to a furnace, differs greatly by region and involves such variables as prevailing costs for fossil fuels, electric rates, and weather conditions. Often, both furnaces and water heaters have to be changed to electric models in order to make the numbers work, so utility companies will have to embrace the technology and push it to their customers aggressively.

For any innovation in the HVAC industry to succeed, sales, distribution, and installation training obstacles have to be overcome, not to mention the kind of manufacturing problems that plagued the first generation of LTHPS that made it into the field. Probably, the hardest thing to overcome is cultural: It’s simply the reluctance of both utility companies and consumers to place their trust in a new product, even if the technologies that made it possible aren’t new. Hopefully, the optimism that inspired David Shaw to come this far will continue to encourage him and his company to keep trying.