Solar Combisystem
Introduction and Definitions :-
Solar combisystems are solar heating installations providing space heating as well as domestic hot water for the inhabitants of the building. The primary energy sources are solar energy as well as an auxiliary source such as biomass, gas, oil and electricity, either direct or with a heat pump. The solar contribution, i.e. the part of the heating demand met by solar energy varies from 10 percent for some systems up to 100% for others, depending on the size of the solar collector surface, the storage volume, the heat load and the climate.
Solar Combisystem France, Direct Solar Floor
In this task the term “solar combisystems” is used only for detached one-family houses, groups of one-family houses, or multifamily buildings with their own heating installations. It does not refer to solar district heating systems or systems with seasonal storage or central solar heating plants with seasonal storage. Solar combisystems generally consist five elements: a solar collector loop; a storage subsystem; a control subsystem; an auxiliary subsystem; and a heat distribution subsystem. Solar combisystems do not generally refer to systems with cooling capabilities. In special designs, the storage subsystem can be combined with the heat distribution subsystem, e. g. floor heating.
Solar Combisystem for a two family house in Austria
A solar combisystem is a solar heating system that provides both space heating and hot water from a common array of solar thermal collectors, normally linked to 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 properties via district heating tend to be called central solar heating schemes.
A large number of different types of solar combisystems are produced - over 20 were identified in the first international survey, conducted as part of IEA Task 14 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 in ultra-low energy Passivhaus type buildings; even up to up to 100% where a large seasonal thermal store 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 other renewable energy sources.
During 2001, around 50% of all the domestic solar collectors installed in Austria, Switzerland, Denmark and Norway were to supply combisystems, while in Sweden it was greater. In Germany, 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 in Canada since the mid 1980s.
It has been suggested that in future combisystems might be able to incorporate absorption solar cooling in summer.
Classification:
Following the work of IEA Task 26 (1998 to 2002), solar combisystems can be classified according to two main aspects; firstly by the heat storage category (the way in which water is added to and drawn from the storage tank and its effect on stratification); secondly by the auxiliary heat management category (the way in which non-solar auxiliary heaters are integrated into the system).
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 water and space heating water at different temperatures.
Heat storage category :
A…. No controlled storage device for space heating
B…. Heat 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 management using natural convection in storage tanks and / or between them to maintain stratification to a certain extent.
D… Heat management using natural convection in storage tanks and built-in stratification devices.
B/D Heat 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
Auxiliary heat management categories:
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 loop is fed alternatively by the solar collectors (or a solar water storage tank), or by the auxiliary heater; or there is no hydraulic connection between the solar heat distribution and the auxiliary heat emissions
S…. Serial mode: The space heating 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 integrated into a single prefabricated 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 water from the solar collectors through underfloor central heating pipes embedded in the concrete floor slab. The floor slab is thickened to provide thermal mass and so that the heat from the pipes (at the bottom of the slab) is released during the evening.
Combisystem design:
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 to nomograms (such as the one developed for IEA Task 26) to various computer simulation software of varying complexity and accuracy.
Among the software and packages are CombiSun (released free by the Task 26 team , which can be used for basic system sizing) and the free SHWwin (Austria, in German ). Other commercial systems are available.
Technologies:
Solar combisystems use similar technologies to those used for solar hot water and for regular central heating and underfloor heating, as well as those used in the auxiliary systems - microgeneration technologies or otherwise.
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.
Relationship to low energy building:
By the end of the 20th century solar hot water systems had been capable of meeting a significant portion of domestic hot water in many climate zones. However it was only with the development of reliable low-energy building techniques in the last decades of the century that extending such systems for space heating became realistic in temperate and colder climatic zones.
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 with underfloor heating, but radiators no longer longer need to be grossly oversized to compensate if not. The volume occupied by the equipment also reduces, which also increases the flexibility in its location, which can be of particular importance in individual houses.
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.
Scope and main activities to be undertaken:
System survey, simulation tools and a standardised test procedure are the necessary infrastructure for understanding and supporting the growing market of solar combisystems. A unique classification and definitions are the necessary basis for any discussion and comparison of different system designs. Simulation gives insight in the thermal behaviour of solar combisystems, which is needed for development, and also for designing and sizing such systems. A standardised test procedure will allow for rating solar combisystems under standardised conditions for testing and for performance prediction. This will lead to greater confidence of the end user in this technology.
Task 26 will review, analyse, test, compare, optimise and improve designs and solutions of solar combisystems for:
*detached one family houses
*groups of one family houses, and
*multifamily houses or equivalent in load
with their own heating installations. This Task does not refer to solar district heating systems or systems with seasonal storage or central solar heating plants with seasonal storage.
Companies from several participating countries will be taking part in this work and will help to make the results of the Task more relevant to the solar heating industry.
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Resources:
http://en.wikipedia.org/wiki/Solar_combisystem
http://www.iea-shc.org/task26/