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Asolar combisystem provides bothsolar space heating andcooling as well ashot water from a common array ofsolar thermal collectors, usually backed up by an auxiliary non-solar heat source.
Solar combisystems may range in size from those installed in individual properties to those serving several in a block heating scheme. Those serving larger groups of propertiesdistrict heating tend to be calledcentral solar heating schemes.
Many types of solar combisystems are produced - over 20 were identified in the first international survey, conducted as part ofIEA SHC Task 14[1] in 1997. The systems on the market in a particular country may be more restricted, however, as different systems have tended to evolve in different countries. Prior to the 1990s such systems tended to be custom-built for each property. Since then commercialised packages have developed and are now generally used.
Depending on the size of the combisystem installed, the annual space heating contribution can range from 10% to 60% or more inultra-low energyPassivhaus-type buildings; even up to 100% where a largeinterseasonal thermal store or concentrating solar thermal heat is used. The remaining heat requirement is supplied by one or more auxiliary sources in order to maintain the heat supply once the solar heated water is exhausted. Such auxiliary heat sources may also use otherrenewable energy sources (when ageothermal heat pump is used, the combisystem is called geosolar)[2] and, sometimes,rechargeable batteries.
During 2001, around 50% of all the domestic solar collectors installed inAustria,Switzerland,Denmark, andNorway were to supply combisystems, while inSweden it was greater. InGermany, where the total collector area installed (900,000 m2) was much larger than in the other countries, 25% was for combisystem installations. Combisystems have also been installed inCanada since the mid-1980s.
Some combisystems can incorporatesolar thermal cooling in summer.[3]
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Following the work ofIEA SHC Task 26 (1998 to 2002), solar combisystems can be classified according to two main aspects; firstly by theheat (or cool) storage category (the way in which water is added to and drawn from thestorage tank and its effect onstratification); secondly by theauxiliary heat (or cool) management category (the way in which non-solar-thermal auxiliary heaters or coolers can be integrated into the system).[4]
Maintaining stratification (the variation in water temperature from cooler at the foot of a tank to warmer at the top) is important so that the combisystem can supply hot or cool water and space heating and cooling water at different temperatures.
| Category | Description |
|---|---|
| A | No controlled storage device for space heating and cooling. |
| B | Heat and cool management and stratification enhancement by means of multiple tanks and/or by multiple inlet/outlet pipes and/or by three- or four-way valves to control flow through the inlet/outlet pipes. |
| C | Heat and cool management using naturalconvection in storage tanks and/or between them to maintain stratification to a certain extent. |
| D | Heat and cool management using natural convection in storage tanks and built-in stratification devices. |
| B/D | Heat and cool management by natural convection in storage tanks and built-in stratifiers as well as multiple tanks and/or multiple inlet/outlet pipes and/or three- or four-way valves to control flow through the inlet/outlet pipes. |
| Category | Description |
|---|---|
| M (mixed mode) | The space heating loop is fed from a single store heated by both solar collectors and the auxiliary heater. |
| P (parallel mode) | The space heating and cooling loop is fed alternatively by thesolar collectors (or a solar water storage tank), or by the auxiliary heater or cooler; or there is no hydraulic connection between the solar heat and cool distribution and the auxiliary heat emissions. |
| S (serial mode) | The space heating and cooling loop may be fed by the auxiliary heater, or by both the solar collectors (or a solar water storage tank) and the auxiliary heater connected in series on the return line of the space heating loop. |
A solar combisystem may therefore be described as being of type B/DS, CS, etc.
Within these types, systems may be configured in many different ways. For the individual house they may – or may not – have the storage tanks, controls and auxiliary heater and cooler integrated into a singleprefabricated package. In contrast, there are also large centralised systems serving a number of properties.
The simplest combisystems – the Type A – have no "controlled storage device". Instead they pump warm (or cool) water from thesolar collectors throughunderfloor central heating pipes embedded in the concrete floor slab. The floor slab is thickened to providethermal mass and so that the heat and cool from the pipes (at the bottom of the slab) is released during the evening.
The size and complexity of combisystems, and the number of options available, mean that comparing design alternatives is not straightforward. Useful approximations of performance can be produced relatively easily, however accurate predictions remain difficult.
Tools for designing solar combisystems are available, varying from manufacturer's guidelines tonomograms (such as the one developed forIEA SHC Task 26) to variouscomputer simulation software of varying complexity and accuracy.[citation needed]
Solar combisystems generally useunderfloor heating and cooling[1].
Concentrating solar thermal technology may be used to make the collectors as small as possible.[citation needed]
Solar combisystems use similar technologies to those used forsolar hot water and for regularcentral heating andunderfloor heating, as well as those used in the auxiliary systems -microgeneration technologies or otherwise.[citation needed]
The element unique to combisystems is the way that these technologies are combined, and the control systems used to integrate them, plus any stratifier technology that might be employed.[citation needed]
By the end of the 20th centurysolar hot water systems had been capable of meeting a significant portion of domestic hot water requirements in manyclimate zones. However it was only with the development of reliablelow-energy building techniques in the last decades of the century that extending such systems for space heating became realistic in temperate and colderclimatic zones.[citation needed]
As heat demand reduces, the overall size and cost of the system is reduced, and the lower water temperatures typical of solar heating may be more readily used - especially when coupled withunderfloor heating or wall heating. The volume occupied by the equipment also reduces, which also increases the flexibility of its location.[citation needed]
In common with other heating systems in low-energy buildings, system performance is more sensitive to the number of occupants, room temperature and ventilation rates, when compared to regular buildings where such effects are small in relation to the higher overall energy demand.[citation needed]
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