Even 'net zero energy houses' and, even more extreme, 'energy plus houses' still need heat production. Both concepts typically achieve zero or a plus by producing enough solar electricity from photovoltaic panels to equal or exceed their total yearly energy consumption - calculated on the basis of primary energy. This is by no means to say that these houses do not need energy input. Given the weak solar radiation during the short winter days, the energy production is minimal in winter, when heating demand occurs. So, heat must be produced. Indeed, if electric heating is used, this must be penalized with the same primary energy factor for electricity that made the net zero or positive energy title possible. The challenge is, then, to produce a very small amount of heat with low capital and ecological costs. Several good solutions are available.
The most prevalent solution is a heat pump, using as its heat source the exhaust room air directly or, in temperate climates, a ventilation heat exchanger. Its limitation is that in very cold weather, it must switch to resistance heating. However, during most of the heating system it can deliver from 1 kW of electricity up to 3 kW of heat. If it is coupled to a ground heat exchanger (an anti-freeze solution circulated through a buried pipe circuit), an even higher output is possible.
Wood pellet stoves offer the advantages of using wood as a fuel (CO2 neutral), being highly automated and operating with a high efficiency due to controlled combustion.
For apartment buildings or as a central plant for a row of houses, a condensing gas boiler may be a solution. Nominal efficiencies exceeding 100 per cent are possible. A drawback is that, given the relatively small absolute amount of heat needed, the fixed costs, maintenance and investment costs are high. These can be tolerated better if shared among multiple housing units.
Given the small energy magnitude for space heating, domestic water heating takes on new importance. This end use may even exceed the space heating. Unlike space heating where the target temperature is 20°C to 22°C, domestic water needs to be heated to 50°C and, possibly, periodically to 60°C. This higher 'exergy' requirement (ability of energy to do work) places a higher demand on the heating production system.
For ecological reasons, an obvious solution is a solar thermal system. With only 1 m2 to 2 m2 of collector per person, this proven technology can cover half the water heating demand. While, economically, it can be argued that if the heat production is by a heat pump or wood pellet stove,
Figure I.3 A compact heating system these systems should also heat DHW. By year-round operation, their high capital costs are more quickly amortized. It can also be argued, however, that psychologically it is appealing to shut off these technical systems for half of the year or more and simply use sunshine to make hot water.
Some clients think this through further and decide to increase the collector area - for example, from 6 m2 to 20 m2 or more - and also increase the storage tank from 500 litres to 2000 litres, as well as to supply some of the needed space heating. The expectations, nevertheless, must take into account the shortened heating season of high-performance housing.
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