Your choices so far:
1 Local heating (ind. house); Comfortable indoor climate
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 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) |
| 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 | 1 Local heating (ind. house) |
Houses may either be part of a municipality or they may be isolated, single houses. Nevertheless, those living in the house will want to have a comfortable indoor climate.
A comfortable indoor climate – looking now only at the thermal aspects of climate control – consists of heating during cold days, cooling during hot days and access to hot tap water all days.
For buildings in densely populated municipalities, these services are, or should be, supplied by district heating and cooling systems. With modern insulation materials district heating systems may become economical once the annual heat load in a geographical area exceeds approximately 10 kWh/m2 and year. For any building outside such areas, the only alternative will be to organise the climate control system locally.
Todays' air-conditioning units will provide air-borne heating as well as cooling in one single unit and often such units will be found in the individual rooms in single-family houses, in apartments in apartment houses and again in single rooms in hotels. In office buildings will the ventilation system often be equipped with a central AC-unit.
For the use of renewable energy in combination with AC-units there is mainly one alternative, 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, by a micro-hydropower installation or by a small wind turbine.
The characteristic for single houses is that the variations in energy need may be very different from hour to hour. For example will the fact that one person in the household takes a shower suddenly demand a rise in heat supply to the hot water system and if windows are opened for intensive ventilation the demand for heating suddenly increases radically. Also the opposite is true: A party with 20 participants will suddenly increase the heat supply to the house with about 2-3 kW (one person releases approximately 100 W only in the form of body heat and if they are dancing the heat release increases) and there might suddenly be a demand for cooling!
Though an apartment house in itself constitutes a number of households is the peak need for heat not necessarily equal to the sum of the peak need for heat in the individual households. The reason for this is that chances are that the peak loads in the individual households occur at different times. Though the short-time peak loads will thus tend to even out will the slower daily variations resulting from social life and working hours sum up. Even if the individual households do not have their morning shower at exactly the same time will they all have their showers at about the same hour of the day.
Office buildings will quite frequently have a pronounced need for cooling during office hours because of the excess heat delivered by office electronics and by the fact that the number of persons per m2 in an office building is usually more than the number of people per m2 in homes. The indoor climate in an office building should be adapted to the fact that most people in the building will be sitting most of the day.
Shopping centres, sports centres, schools, hospitals, official buildings and such, aimed to host a large number of people of varying ages and constitutions and not primarily a cadre of sitting middle-aged people, will again pose new demands on the indoor climate control. Central heating in buildings can be water-borne – which is preferred in case of biofuel – or it may be air-borne. In case of water-borne heating, the production of tap water is normally integrated in the same boiler but – of course – with a separate heat exchanger coil.
In case of air-borne heating, the production of tap water becomes a completely separate system.
With water-borne systems in larger buildings, where the thermal load may be anything from 50 – 1000 kW, the water volume in the building heating system will usually be big enough to in itself even out the variations in thermal demand and hence render an accumulator tank unnecessary. With an accumulator tank integrated in the system, though, the integration of solar heating with the system by a separate heat exchanger coil in the accumulator becomes simple.
Since the water volume in the boiler itself – or in the combined boiler and accumulator – is significant, a water-borne system will provide a thermal inertia that simplifies the control of the system. A sudden increase of the hot water demand, for example, can then be supplied from the stored energy in the system so that the demand can be met while the heat input (for example a pellet burner or – in some cases – a wood chip boiler) will need some time to get started. The larger the thermal inertia, the longer start-up times may be accepted. With air-borne heating systems, the thermal inertia is close to zero and the heating system must respond much more rapidly.
In larger buildings, where the thermal inertia of the building structure itself is large, can air-borne heating be combined with pellets and with wood chips firing. The supply of hot water, though, will demand separate a water heating circuit.