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Heat pumps

Category: Systems of interest October 9th, 2008 by mbc

Many people visiting this site are looking for information about heat pumps, being one who believes in giving the people what they want I thought an up-to-date summary was in order…

Heat pumps take heat from a donor heat transfer medium (earth / air / water) and condense it for the purpose of heating water or air.

The basis of heat pump technology is something we are all familiar with as it is similar to that used in refrigerators or air conditioning units ~ but in reverse. The heated water generated by a heat pump is often used for space heating but can also be used as the basis for general hot water provision given an additional boost from a complementary boiler system to raise water temperature.

Heat pumps consume electricity to power the compressor & pump that circulate fluid and gas around the system; this is a relatively efficient use of energy typically producing between two and four units of heat for each unit of electricity consumed. The ratio of energy consumed to heat generated is know as the coefficient of performance (COP). There is a theoretical limit of around 14:1.

What types of heat pump are there?
The air source heat pump (ASHP) draws thermal energy from the air.

The ground source heat pump (GSHP) gathers heat from a ground loop. This is constructed from lengths of pipe filled with a blend of water and antifreeze in a closed loop. This loop is buried in the earth either horizontally in trenches or vertically in a bore hole.

The water source heat pump (WSHP) uses water as the heat transfer medium.

How do they work?

  • A liquid refrigerant is forced through an expansion valve & in doing so it loses pressure and evaporates.
  • In evaporating, the liquid refrigerant removes heat from the input loop. The exact nature of the input loop will vary depending on the type of heat pump as discussed earlier ~ in general terms it brings heat energy to the evaporator coil from the donor medium.
  • Cooled water in the input loop is recirculated and reheated by the transfer medium.
  • The evaporated refrigerant passes through the compressor that increases pressure and causes the refrigerant vapour to condense at an increased temperature.
  • As the refrigerant vapour condenses heat energy is released, this heat raises the temperature of the condenser coil which in turn heats water.
  • This water is then used for space heating.
heat pump parts and process
the basic structure of a heat pump

Where should I use a heat pump?
They are best coupled with well insulated, energy efficient buildings with an under floor heating system for heat distribution. Traditional radiators can be used, but will need to be larger in total surface area than those found as part of a conventional system due to the lower water heating temperatures, such a system will also be less efficient than an underfloor alternative. Air source heat pumps are often used in conjunction with ducted air heating systems.

You will need plenty of outdoor space for a horizontally installed GSHP ground loop. Alternatively, a vertical bore hole can be sunk but will cost more.

Bear in mind that the payback period will be longer if the heat pump is replacing gas rather than other heating options such as solid fuel or electricity. However, with soaring gas prices the payback period is shortening.

The higher the temperature that water is heated to, the lower the COP of the pump will be. Bear this in mind in designing your overall hot water system, providing additional inputs to raise water temperatures to domestic hot water (washing) levels.

How much do they cost?
Costs have remained relatively static over the last year, a 6kW ASHP costs around £3,500 with a larger 12kW pump coming in at about £6,000. You will them need to pay for installation and excavation, this will potentially double costs, so allow £6,000 to £12,000 for a domestic installation. This excludes the cost of the distribution system such as an under floor heating.

Summary
Future developments, such as photovoltaic cells with a shorter payback time or more exotic combinations for example, bio-fuel driven Micro Combined Heat and Power (micro-CHP) installations providing input electricity, offer greater levels of sustainability and self-sufficiency in heat pump utilisation. This doesn’t stop heat pumps from offering a green heating solution today, with guaranteed CO2 savings over traditional heating systems and a relatively fast payback where mains gas is not available.

Other Resources
Grants may be available for UK installations see http://www.lowcarbonbuildings.org.uk/micro/
Proceed with caution where gas is available according to carbonlimited.org

Previous Heat Pump Articles on MBC
GSHP
ASHP

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