Solar Energy Perspectives - IEA

Solar Energy Perspectives: Buildings
Most heat pumps are run from electricity. They introduce more heat in the building than an
electric heater would do. An electric heater would never convert more than 100% of
electricity into heat. Heat pumps do, however, because they “pump” the heat from the cold
outside to the warm inside. Their coefficient of performance (CoP) measures the ratio of heat
transferred to consumption of the pump. An important measure is the annual average CoP,
also called the seasonal performance factor (SPF). The CoP varies considerably with the
design of the whole system, and its conditions of use, but the basic principle is simple: the
greater the rise in temperature, the lower the CoP. For example, a heat pump using outer air
at 0°C and feeding small radiators, originally designed for some boiler with water at 60°C,
would have a CoP of only 1.5 to 2. If the lift is even greater, the CoP may fall below unity.
Indeed most of the energy is then brought in by transforming into heat the work of the pump,
which is an inefficient and costly way of making heat from electricity. By comparison,
a ground-source heat pump using heat from the soil at 12°C and feeding a heating floor (with
a large heat exchange area) with water at 35°C, can exceed CoP of 6. The implicit assumption
in Figure 4.7 is of a CoP of 4, which would be the SPF of good domestic ground-source or
larger air-source heat pumps in cold climates. Of course, the heat pump itself must be well
designed and run smoothly with an electronic inverter and variable speed, not simply an
on-off device.
Which heat pumps justify the term “renewable energy”, and should all the heat they deliver
be considered renewable? The European Union stated its position in the Directive of the
European Parliament and the Council of 23 April 2009 on renewable energy. First, heat
pumps are considered renewable provided that the final energy output significantly exceeds
the primary energy input required to drive the heat pumps. In practice, electric heat pumps
would need to have an SPF greater than the ratio of the primary consumption for electricity
production. This has been calculated as an EU average, over the total gross production of
electricity in Europe, plus 15%, i.e. greater than 2.875 with the current electricity mix. The
energy considered renewable is the heat delivered, minus the electricity consumption of the
pump. The share of renewable energy in any given heating system increases with better SPF.
In cold climates, ground-source heat pumps should be preferred and their working conditions
optimised. In new buildings, the choice of heating floors is easy. In renovations, better
insulation could allow existing small radiators to heat the interior with a smaller working
temperature than before, but increasing the radiator area would still be advisable to further
reduce this working temperature.
In the ground, temperature conditions are quite stable all year round, because the heat is very
slow to move through the soil and the renewable energy from either above (the sun) or below
(the earth’s interior warmth) keeps temperatures roughly constant. However, because the heat
is so slow to move through the soil, in the immediate neighbourhood of the boreholes and
pipes from the heat pump, temperatures will progressively diminish during the winter as the
heat pump locally removes the heat. A colder area is thus created, and the CoP of the heat
pump will progressively diminish as well, thereby affecting the SPF.
There are several options to avoid this. One is to warm the working fluid by a few degrees
before it enters the heat pump. This can be done by a relatively small solar collector surface
area. Another is to inject heat into the ground in summer, so as to start the heating season
with a higher temperature around the boreholes. This can be done by cooling the building in
summer, either reversing the heat pump (simple valves can do this), or making the fluid in the
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© OECD/IEA, 2011