Heat pump Thermal efficiency
Thermal efficiency
The net thermal efficiency of a heat pump should take into account the efficiency of electricity generation and transmission, typically about 40%. Since a heat pump takes heat from the ground, the total heat energy output to the building is greater than the electricity input. This results in thermal efficiencies greater than 100%, up to around 150%.
Net thermal efficiency tends to be confusing to consumers, so heat pump performance is generally expressed as the ratio of heat output to electricity input. An allowance is included for electricity used by the fluid pumps. Cooling performance is typically expressed in units of BTU/hr/Watt as the Energy Efficiency Ratio, (EER) while heating performance is typically reduced to dimensionless units as the Coefficient of Performance. (COP) The conversion factor is 3.41 BTU/hr/Watt. Both of these measures will vary depending on the temperature difference between the ground source and the building, which can vary greatly between installations and over the course of the year.
For the sake of comparing ground source heat pump appliances to each other, independent of installation variations, a few standard test conditions have been established by the American Refrigerant Institute (ARI) and more recently by the International Standards Organization. Standard ARI 330 ratings were intended for closed loop ground-source heat pumps, and assumes secondary loop water temperatures of 77°F for air conditioning and 32°F for heating. These temperatures are typical of installations in the northern USA. Standard ARI 325 ratings were intended for open loop ground-source heat pumps, and include two sets of ratings for groundwater temperatures of 50°F and 70°F. ARI 325 budgets more electricity for water pumping than ARI 330. Neither of these standards attempt to account for seasonal variations. Standard ARI 870 ratings are intended for direct exchange ground-source heat pumps. ASHRAE transitioned to ISO 13256-1 in 2001, which replaces ARI 320, 325 and 330. The new ISO standard produces slightly higher ratings because it no longer budgets any electricity for water pumps.
Residential ground source heat pumps on the market today have COP's ranging from 2.4 to 5.0 and EER's ranging from 10.6 to 30. To qualify for an Energy Star label, heat pumps must meet certain minimum COP and EER ratings which depend on the ground heat exchanger type. For closed loop systems, the ISO 13256-1 heating COP must be 3.3 or greater and the cooling EER must be 14.1 or greater.
Many factors effect the efficiency of ground source heat pumps. COP improves with a lower delta-T, the temperature difference between the input and output of the heat pump. The volume of ground that is used must be large enough that the heat addition or subtraction does not cause a significant change in temperature. Efficient compressors, variable speed compressors and larger heat exchangers produce more efficient heat pumps. Soaker hoses are sometimes used to wet the ground to increase heat transfer.
Soil without artificial heat addition or subtraction and at depths of several meters or more remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location. It is usually 7-12°C (45-54°F) at a depth of six meters in locations where heating is needed in winter. Ground-source heat pumps rely on this near constant temperature as a base temperature that is raised or lowered minimally to create a desirable indoor temperature. The soil temperature will vary as heat is added or removed. Because this temperature remains more constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency and are stressed less during extreme air temperatures than fueled or electric conventional air conditioners and furnaces. A particular advantage is that they can use electricity to heat spaces and water much more efficiently than an electric heater.
Seasonal variations are much more important for air-source heat pumps, and ARI 210 and 240 define Seasonal Energy Efficiency Ratios (SEER) and Heating Seasonal Performance Factors (HSPF) to take into seasonal variations into account for these units. These numbers are normally not applicable and should not be compared to ground-source heat pump ratings. However, Natural Resources Canada has adapted this approach to calculate typical seasonally adjusted HSPF's for ground-source heat pumps in Canada. The NRC HSPF's ranged from 8.7 to 12.8 BTU/hr/Watt (2.6 to 3.8 in nondimensional factors, or 255% to 375% seasonal average electricity utilization efficiency) for the most populated regions of Canada. When combined with the thermal efficiency of electricity, this corresponds to net average thermal efficiencies of 100% to 150%.
Economics
Ground-source heat pumps are characterised by high capital costs and low operational costs compared to other HVAC systems. Their overall economic benefit depends primarily on the relative costs of electricity and fuels, which are highly variable over time and across the world. Based on recent prices, ground-source heat pumps currently have lower operational costs than any other conventional heating source almost everywhere in the world. Natural gas is the only fuel with competitive operational costs, and only in a handful of countries where it is exceptionally cheap, or where electricity is exceptionally expensive. In general, a homeowner may save anywhere from 20% to 60% annually on utilities by switching from an ordinary system to a ground-source system.
Captical costs and system lifespan have received much less study, and the return on investment is highly variable. One study found the total installed cost for a system with 10kW (3 ton) thermal capacity for a detached rural residence in the USA averaged $8000–$9000 in 1995 US dollars. A more recent study found an average cost of $14,000 in 2008 US dollars for the same size system in Indiana. Prices over $20,000 are quoted in Canada, with one source placing them in the range of $30,000-$34,000 Canadian dollars. The rapid escalation in system price has been accompanied by rapid improvements in efficiency and reliability. Capital costs are known to benefit from economies of scale, particularly for open loop systems, so they are more cost-effective for larger commercial buildings and harsher climates. The initial cost can be two to five times that of a conventional heating system in most residential applications, new construction or existing. In retrofits, the cost of installation is affected by the size of living area, the home's age, insulation characteristics, the geology of the area, and location of the home/property. Proper duct system design and mechanical air exchange should be considered in the initial system cost.
Capital costs may be offset by substantial subsidies from many governments, for example totalling over $7000 in Ontario for residential systems installed in the 2009 fiscal year. Some electric companies will offer special rates to customers who install a ground-source heat pump for heating/cooling their building. This is due to the fact that electrical plants have the largest loads during summer months and much of their capacity sits idle during winter months. This allows the electric company to use more of their facility during the winter months and sell more electricity. It also allows them to reduce peak usage during the summer (due to the increased efficiency of heat pumps), thereby avoiding costly construction of new power plants. For the same reasons, other utility companies have started to pay for the installation of ground-source heat pumps at customer residences. They lease the systems to their customers for a monthly fee, at a net overall savings to the customer.
The life span of the system is longer than conventional heating and cooling systems. Good data on system lifespan is not yet available because the technology is too recent, but many early systems are still operational today after 25–30 years with routine maintenance. Most loop fields are warrantied for 25 to 50 years and are expected to last at least 50 to 200 years. Ground-source heat pumps use electricity for heating the house. The higher investment above conventional oil or electric systems may be returned in energy savings in 2–10 years for residential systems in the USA. If compared to compared to natural gas systems, the payback period can be much longer. The payback period for larger commercial systems in the USA is 1-5 years, even when compared to natural gas.
Ground-source heat pumps are recognized as one of the most efficient heating and cooling systems on the market. They are often the second-most cost effective solution in extreme climates, (after co-generation,) despite reductions in thermal efficiency due to ground temperature. (The ground source is warmer in climates that need strong air conditioning, and cooler in climates that need strong heating.)
Commercial systems maintenance costs in the USA have historically been between $0.11 to $0.22 per m2 per year in 1996 dollars, much less than the average $0.54 per m2 per year for conventional HVAC systems.
Governments that promote renewable energy will likely offer incentives for the consumer (residential), or industrial markets. For example, in the United States, incentives are offered both on the State and Federal levels of government. From Wikipedia, the free encyclopedia