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Making the optimum use of the power Heat pumps make efficient use of electrical energy and can work in Alaska’s climate for cost effective heating of buildings ALAN BAILEY Petroleum News
The heat pump, a technology that has been around for many years, is best known to many people as the engine that drives a household refrigerator or air conditioning system. But the technology also works as a means of heating a building, an application that can work effectively even in Alaska’s cool and often cold climate. In fact, depending on factors such as the cost of the alternative form of heating and the local price of electricity, a heat pump can prove financially attractive, in addition to being an especially clean heating mechanism.
A heat pump in a refrigerator, for example, involves a liquid refrigerant evaporating to a gas in a system of tubes inside the refrigerator, with the absorbed latent heat of evaporation causing the refrigerator to cool. A pump then compresses the gas back to a liquid in tubes outside the refrigerator, releasing latent heat to the outside of the appliance. The compressed liquid flows back into the refrigerator to be evaporated again. As this process continues, with the refrigerant cycling around the system, heat is continuously transferred, or pumped, from the inside of the appliance to the outside.
Now imagine transferring this technology to a building, but reversing the process so that the refrigerant evaporation takes place outside the building, while the compression happens inside. The result will be the heating of the building by pumping heat from outside to inside. The heat pump itself can be entirely self contained within the building, with a fluid such as water used to transfer energy from outside the building into the cool side of the pump, or used to transfer heat around the building from the pump’s warm side.
Efficient electricity use Sean Skaling, policy and program director for the Alaska Energy Authority, told Petroleum News that the key advantage of a heat pump is the manner in which it can place more heat in a building than is represented by the electrical energy that the pump consumes. The ratio of the energy pumped into the building to the electrical energy used is called the coefficient of performance, or COP for short. A traditional electrical heater, using a heating element, uses most of the electricity it consumes to generate an equivalent amount of heat and, therefore, has a COP of around one. But a heat pump, with a COP that can be three or more, is much more efficient in its use of electrical power, Skaling explained.
The Alaska Energy Authority has been promoting the use of heat pumps in Alaska in situations where this approach can work technically and financially. There are now a number of locations in the state where the technology has proved successful in lowering heating bills, Skaling said. The economics depend on relatively cheap electricity, the opportunity to replace an expensive alternative heating source, and a workable source of heat for the pump. Heat sources can be the outside air, a water body or the ground. The source does not have to be hot, or even warm, but it does need some combination of a high heat capacity and a means of heat replenishment to ensure a stable, continuing heat flow.
Andy Baker, a clean energy consultant, told Petroleum News that for maximum efficiency a heat pump needs to run overloaded, trying to move more heat than its capacity. In a cold climate such as Alaska’s this factor can be used to advantage, with perhaps several pumps being run flat out, and with some pumps shutting down if the heating load drops. Baker also emphasized that, rather than simply buying a heat pump and installing it, it is necessary to custom design the various fluid loops and the heat source required for a particular application, to ensure a cost-effective implementation.
And what can be relatively high up-front installation costs need to be factored into the overall project economics.
The Alaska SeaLife Center One of the higher profile heat pump applications in Alaska is in the Alaska SeaLife Center in Seward, where a system of heat pumps now meets all of the building’s heating needs. The center uses seawater from Resurrection Bay as a heat source, a convenient arrangement since the water is already being pumped through the various marine life installations in the center.
Fortuitously, thanks to ocean currents from the south, the water in Resurrection Bay at Seward is relatively warm, reaching a peak temperature of 56 degrees F in November and cooling to a minimum of around 40 degrees F by May, Baker said.
Darryl Schaefermeyer, special projects director for the center, told Petroleum News that the very high cost of heating oil in 2008 had driven the center’s interest in converting from oil-fired heating systems to heat pumps. With total funding support of $713,000 from the Alaska Energy Authority’s Renewable Energy Fund and the Denali Commission, the SeaLife Center installed two heat pumps. Those pumps started operating in 2011, sending heat to the center’s five largest air handlers and pre-heating the domestic hot water.
A grant from the Murdock Charitable Trust enabled an extension of the system into pavement heating, a major source of heat demand for the center.
The most recent upgrade, funded by a $537,000 AEA grant matched by $68,000 in funding from the SeaLife Center, has involved the installation of four additional state-of-the-art heat pumps that use carbon dioxide as refrigerant. The phase-change properties of carbon dioxide allow this type of pump to transfer heat at a temperature of up to 190 F, a much higher temperature level than the maximum of 120 F from a traditional refrigerant. That has enabled the full heat-pump heating of the center’s hot water supply, as well the ability to use the carbon dioxide pumps for the building’s medium temperature heating.
Eliminated heating oil The upshot has been the elimination of the need to purchase heating oil for the SeaLife Center. Prior to the heat pump installation, the building was consuming between 100,000 and 130,000 gallons of heating oil per year, Schaefermeyer said. Although the price of fuel oil has dropped since then, the comparison in energy costs before and after heat pump installation remains impressive: In 2008 the center’s combined electricity and fuel bill was $1.2 million, while in 2015, with no heating fuel needed, the electricity bill amounted to a little over $500,000, Schaefermeyer said.
Elsewhere on the Kenai Peninsula, there has been a particularly successful heat pump application in a senior housing complex in Seldovia, Baker said. This system, which has displaced about 90 percent of the building’s oil boiler usage, involves heat extracted from a layer of basalt rock under the parking lot adjacent the building. HDPE (high-density polyethylene) piping carrying fluid for extracting heat from the rock passes through 10 holes drilled 300 feet vertically through the rock, Baker said.
Juneau Airport There is another high-profile heat pump application in the Juneau airport in Southeast Alaska. Like the Seldovia application, the airport system uses ground-sourced heat pumps, using piping run through holes drilled into the subsurface. The availability of modestly price hydropower in Southeast Alaska can make the use of heat pumps particularly appealing in that region.
Construction of the Juneau airport system began in 2009 in conjunction with a major renovation of the airport terminal that took place at that time. An AEA grant of $513,000, half of the total project cost, supported the heat pump project. The heat source involves HDPE piping that passes through 108 vertical holes bored beneath the commuter airplane ramp adjacent the terminal. A total of more than 16 miles of the piping circulates a mixture of water and methanol, transferring heat energy from the ground into more than 30 electrically powered heat pumps inside the terminal. In addition to providing heat for the renovated section of the building, the system, which was completed in early 2011, heats a snow melt system for the front sidewalk, street crossing and a waiting area.
An analysis of the operation of the system, published in 2014, indicates a direct savings of about $130,000 per year, with a payback period on installation costs of less than eight years.
But are heat pumps a feasible option in colder parts of Alaska, farther north?
Project in Fairbanks The Cold Climate Housing Research Center has been conducting a research project in Fairbanks in the Alaska Interior, using a ground source heat pump system to heat a building to test the feasibility of heat pump use in an area of semi-continuous permafrost - a six-ton residential heat pump, installed in 2011, replaced a 76,000-Btu-per-hour oil fired boiler. The heat pump delivers heat to the existing in-floor water-based heat distribution system. For a heat source, the project involved the horizontal placement of 4,800 lineal feet of coiled tubing a few feet underground, outside the building, below the active soil layer and above the top of the permafrost. Water mixed with methanol antifreeze flows through the tubing, transferring heat energy from the ground into the intake side of the heat pump.
A concept behind the system is that in the depth of the winter the ground temperature a few feet below the surface remains much higher than the temperature of the frigid air above, thus providing a temperature contrast that can drive a heat pump efficiently.
Robbin Garber-Slaght, a research engineer in the CCHRC, told Petroleum News that the system performance was excellent during the first year of operation, with a COP of 3.7. Over the subsequent two years the performance dropped, as the ground around the buried tubing cooled under the refrigerating effect of the system. But after three years the system was still delivering a respectable COP of around 2.9 during the winter. Garber-Slaght said that the temperature around the underground tubing has now dropped to about 32 F but that, theoretically, the heat pump would remain efficient down to 20 F.
As part of the research project, the CCHRC team is testing the effect of different ground covers, such as sand or grass, on the efficiency of the system. The idea is that, although the system will cool the subsurface ground when in use during the winter, the summer heat will warm the ground back up again. Another possibility would be to reverse the heat pump in the summer, using the pump to cool the building by delivering heat back into the ground.
Garber-Slaght said that the original three-year AEA grant for funding the project is coming to an end that the team hopes for further funding to determine the longer term performance of the heat pump system, and to figure out whether the continuing heat flow through the system has reached an efficiency equilibrium.
The CCHRC has also been conducting a project to test the efficiency of air sourced heat pumps in Southeast Alaska.
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