Your choices:
1 Comfortable indoor climate; 2 Sunshine; 3 Local heating (ind house)
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 1 Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| 2 Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | 3 Local heating (ind. house) |
The first and foremost way to use solar energy to provide space heating in individual buildings will be through passive heating. Making use of passive heating in a building is a question of the building architecture and of the general layout of the building.
As a complement to passive heating, solar collectors for hot-water production may be placed on the rooftop to provide space heating and tap water. Using the simplest, and cheapest, flat-plate, glazed, collectors may provide water at 30-35 °C during sunny winter days while the temperature may reach 80 °C during summer. For tap-water production, where temperatures must exceed 50 °C for sanitary reasons, the winter temperature is obviously not enough while during summer the temperature may well exceed the safety limit for scalding.
With the more advanced vacuum-tube solar collectors you will reach higher but more stable temperatures, but the price paid is that they are significantly more expensive.
The third and most advanced option is to use a low-temperature solar energy collector, such as a water-tube immersed 30-50 cm under-ground or in a pond, and then to make use of a heat pump to raise the temperature to the desired level for tap-water production and for space heating.
With the two first system solutions, the area for the solar collectors is usually restricted to the roof area of the house and with multi-storey buildings this will usually not be sufficient for the energy needs in the building. With a one-storey, single-family house, though, may solar heating well provide the main part of the annual energy needs for heating.
For the use of renewable energy in combination with AC-units there is mainly one alternative for buildings outside the areas where district heating and cooling is supplied, and that is to provide at least part of the electricity need for the house by local, individual, generation. This can be achieved by solar cells. If situated in a sunny climate – and the cooling needs will always be most pronounced when the sun is shining bright – roof-mounted solar cells may well provide a significant amount of the marginal electricity needed for cooling.
Solar cell electricity production suffers mainly from three limitations:
- First the intensity of solar irradiation is limited and only scarcely exceeds 800 W/m2 as measured on a surface oriented at a right angle to the incoming sunlight.
- Second the sun does not always shine bright and thus the intensity just mentioned is only reached during a limited number of hours.
- Third the actual efficiency with commercial solar cells is today less than 20%.
The net result of this is that only scarcely can more than 160 W of electricity be produced per m2 of solar cell area. But on the other hand, this electricity will be synchronized with the need for cooling purposes. Assuming 6 h at a mean production of 150 W/m2, the total will be 0.9 kWh/m2 which may – provided a large enough array – well be a significant contribution.
The use of solar-driven, absorption heat pumps for individual house cooling will be costly, though it is an alternative that is technically available.