Heat Exchangers/Heat Pumps

The term Heat Exchanger applies to any system that allows heat to be exchanged between substances; whereas the term Heat Pump, strictly speaking, refers only to a device that uses some energy to extract a large amount of heat from any substance that contains a low level of heat. However, the two terms are frequently used synonymously.

The earliest Heat Exchangers were undoubtedly Stones heated in a fire and then transferred to a bowl of cold water in order to heat it by exchanging their heat for its coldness. They were then removed and the procedure was repeated. To-day we have come a long way and one can get a Heat Exchanger for almost any situation where one wants e.g. to heat cold air entering a building by removing heat from the warm air leaving the building. NB Air circulation is very important in all buildings.

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How might it work for you

Probably the simplest form of Heat Exchanger, used in our houses today, is Air. In the 1970s alternative technologists researched the effectiveness of Passive Solar Walls. These were simply a south facing brick/stone wall with a sheet of glass i.e. a large 'false' window over the outside, an air space of ca. 30cms between the bricks and the glass, a cold air outlet from the house through the bricks/stone at the base of the wall, and an hot air inlet from the air-filled space into the house at the top of the Solar Wall. Both inlet and outlet were fitted with a closable flap to prevent the system going into reverse during the night and early morning. This system means that the inside room heats from the top down and there are no nasty draughts to chill one's feet. To-day's Sunrooms can be used in exactly the same way. Even with only short bursts of sunlight the temperature of a Sunroom on the South side of a house can rise quite quickly even in the winter. It can also be worthwhile sited on the West side, but is virtually useless in winter on the East and North sides of the house.

With the discovery of how to make a refrigerator, came the ability to use a variation of the Air Heat Exchanger. By using the expansion and contraction of an enclosed gas, heat could be removed or generated, and it is this process that has been developed for the machines known as Heat Pumps.

Note that whereas a Heat Exchanger does not require any extra source of energy other than that needed to open a door or flap; the Heat Pump does require electrical energy to operate the pump in order to compress the gas/air into a liquid state and to circulate the heating fluid - usually oil or water with antifreeze. This means that you may be able to consider utilizing Solar photovoltaic cells, or a small wind turbine, or a combination of these, or a water turbine, or a Biofuel generator, in order to produce and store enough electricity to run the Heat Pump (this includes an Extractor). Combined Heat and Power systems using Biomass are also in development, but as yet their large size makes them generally unsuitable for domestic use.

The Modern Heat Exchangers/Heat Pumps can build up useful temperatures in a liquid that is then circulated to heat the inside of a building starting from really quite low temperatures. Even air at temperatures as low as -15°C has some useful heat that can be abstracted, though at -18°C this method becomes pointless as the amount of electricity used to compress the gas is greater than the kWh equivalents of the heat obtained. This means that Heat Pump systems are rated according to a Coefficient of Performance (CoP) see below.

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Facts - Energy Produced and Costs of Production

The energy produced will vary according to the amount of heat that can be obtained from the air, ground or water. Basically this states the balance between electricity used and the electrical equivalent of the heat gained from the air etc. e.g. an Electric Fire would have a CoP of 1 - 0 as all the energy is being supplied by electricity and on a mild day an Air Heat Exchanger would have a CoP of 1 - 3 or 4. i.e. 1kWh of electricity is used to produce 3 or 4 kWh of heat from the air.

On a cold day the Heat Exchanger, using Air Source, has to work harder in terms of taking in more air to abstract the same amount of heat to be passed to the heating system and this means that the CoP falls to 1 - 2 or <2. If the incoming air is cold enough, the CoP may even fall to 1 - 1 at which point one might as well be using electricity on its own.

One way of avoiding the use of very cold air is to ensure that the system is set up on a south facing wall and it is essential to include an Accumulator Tank in the set up, to hold a reserve of heated fluid ready for circulation. This is placed in the system between the Exchanger and the circulation pump. Thus on a very cold morning, or overnight, it should be possible to use the circulating part of the system from the Accumulator Tank without switching on the Heat Exchanger to draw cold air in, until the outside air temperature has warmed up. Providing that the Accumulator Tank and all the piping is well lagged, this will mean that the building can get a quick heat fix when people need it most.

It is said that Air Source Heat Pumps can deliver up to 16kW of heat equivalent, but the reference does not state the season or the temperature of the incoming air!

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Pros and Cons

Pros:

  • The internal space needed to house an Heat Exchanger is the same for all the Heat Pump systems. It is an essential part of any Heat Pump system and is usually installed as a part of the overall package.
  • This allows one to use heat from the natural environment - Air, Earth, Bed-rock or Water.
  • Electricity from Photovoltaic Panels or a Wind or Water Turbine, could be used to run the Heat Exchanger.
  • The electrical equivalent of the heat gained under normal circumstances is always greater than the electricity used by the Heat Exchanger.

Cons:

  • The compressor does require electricity to run.
  • This system frequently results in a hotter temperature throughout the house, which may not actually be required. If not, then traditional radiators could be used to provide focal points of heat in a room.
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How and Why it works

A simple Heat Pump consists of a closed loop of piping containing a fluid that vaporizes quickly when passed into a larger diameter of pipe/chamber, and liquidizes again when compressed by being pumped into a smaller diameter pipe. Basic chemistry explains this phenomenon by noting that in a substance in its gaseous state the individual atoms/molecules are far apart and rarely come into contact. However, as they are compressed, into a smaller volume, increasing numbers of the molecules collide; the individual atoms are also jostled and the result is that the electrons surrounding the atoms move from an higher to a lower energy level releasing heat in the process. In another way of thinking of it, the electrons are pushed back closer to their nucleus, and like toddlers running free and then constrained whilst their mothers talk, they cool down and their heat warms the air around them. As soon as mother moves on and they are released, they run around and start to heat up again, due in this instance, to the biochemical reactions inside them. The heat released by this process can then be passed on into a circulating fluid in order to heat radiators under floor heating pipes etc.

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Top Tips new and used

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