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A solar combisystem provides both solar space heating and cooling as well as hot water from a common array of solar 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 properties district heating tend to be called central solar heating schemes.
Many types of solar combisystems are produced - over 20 were identified in the first international survey, conducted as part of IEA 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 in ultra-low energy Passivhaus -type buildings; even up to 100% where a large interseasonal 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 other renewable energy sources (when a geothermal 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 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.
Some combisystems can incorporate solar thermal cooling in summer. [3]
Following the work of IEA SHC Task 26 (1998 to 2002), solar combisystems can be classified according to two main aspects; firstly by the heat (or cool) 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 (or cool) management category (the way in which non-solar-thermal auxiliary heaters or coolers can be 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 or cool water and space heating and cooling water at different temperatures.
Category | Description |
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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 natural convection 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 |
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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 the solar 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 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 (or cool) 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 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 to nomograms (such as the one developed for IEA SHC 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, [4] which can be used for basic system sizing) and the free SHWwin (Austria, in German [5] ). Other commercial systems are available.
Solar combisystems generally use underfloor heating and cooling .
Concentrating solar thermal technology may be used to make the collectors as small as possible.
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.
By the end of the 20th century solar hot water systems had been capable of meeting a significant portion of domestic hot water requirements 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 or wall heating. The volume occupied by the equipment also reduces, which also increases the flexibility of its location.
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.
Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy, and solar architecture. It is an essential source of renewable energy, and its technologies are broadly characterized as either passive solar or active solar depending on how they capture and distribute solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power, and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light-dispersing properties, and designing spaces that naturally circulate air.
In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.
A heat pump is a device that can heat a building by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing heat from the enclosed space and rejecting it outside. Units that only provide cooling are called air conditioners.
Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in industry, and in the residential and commercial sectors.
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses.
Solar water heating (SWH) is heating water by sunlight, using a solar thermal collector. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. SWHs are widely used for residential and some industrial applications.
A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and solar towers or non water heating devices such as solar cooker, solar air heaters.
Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, building, multiuser-building, district, town, or region. Usage examples are the balancing of energy demand between daytime and nighttime, storing summer heat for winter heating, or winter cold for summer air conditioning. Storage media include water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and insulated at the top, as well as eutectic solutions and phase-change materials.
Electric heating is a process in which electrical energy is converted directly to heat energy at around 100% efficiency, using rather cheap devices. Common applications include space heating, cooking, water heating and industrial processes. An electric heater is an electrical device that converts an electric current into heat. The heating element inside every electric heater is an electrical resistor, and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy. Most modern electric heating devices use nichrome wire as the active element; the heating element, depicted on the right, uses nichrome wire supported by ceramic insulators.
Renewable heat is an application of renewable energy referring to the generation of heat from renewable sources; for example, feeding radiators with water warmed by focused solar radiation rather than by a fossil fuel boiler. Renewable heat technologies include renewable biofuels, solar heating, geothermal heating, heat pumps and heat exchangers. Insulation is almost always an important factor in how renewable heating is implemented.
Underfloor heating and cooling is a form of central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor. Heating is achieved by conduction, radiation and convection. Use of underfloor heating dates back to the Neoglacial and Neolithic periods.
Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility than the original energy source. Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation.
Seasonal thermal energy storage (STES), also known as inter-seasonal thermal energy storage, is the storage of heat or cold for periods of up to several months. The thermal energy can be collected whenever it is available and be used whenever needed, such as in the opposing season. For example, heat from solar collectors or waste heat from air conditioning equipment can be gathered in hot months for space heating use when needed, including during winter months. Waste heat from industrial process can similarly be stored and be used much later or the natural cold of winter air can be stored for summertime air conditioning.
A ground source heat pump is a heating/cooling system for buildings that uses a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through the seasons. Ground source heat pumps (GSHPs) – or geothermal heat pumps (GHP) as they are commonly termed in North America – are among the most energy-efficient technologies for providing HVAC and water heating, using far less energy than can be achieved by burning a fuel in a boiler/furnace or by use of resistive electric heaters.
Central solar heating is the provision of central heating and hot water from solar energy by a system in which the water is heated centrally by arrays of solar thermal collectors and distributed through district heating pipe networks.
SolarPACES is an international cooperative network bringing together teams of national experts from around the world to focus on the development and marketing of Concentrating Solar Power (CSP) systems.
Photovoltaic thermal collectors, typically abbreviated as PVT collectors and also known as hybrid solar collectors, photovoltaic thermal solar collectors, PV/T collectors or solar cogeneration systems, are power generation technologies that convert solar radiation into usable thermal and electrical energy. PVT collectors combine photovoltaic solar cells, which convert sunlight into electricity, with a solar thermal collector, which transfers the otherwise unused waste heat from the PV module to a heat transfer fluid. By combining electricity and heat generation within the same component, these technologies can reach a higher overall efficiency than solar photovoltaic (PV) or solar thermal (T) alone.
Solar air heating is a solar thermal technology in which the energy from the sun, insolation, is captured by an absorbing medium and used to heat air. Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications. It is typically the most cost-effective out of all the solar technologies, especially in commercial and industrial applications, and it addresses the largest usage of building energy in heating climates, which is space heating and industrial process heating.
The International Energy Agency Solar Heating and Cooling Technology Collaboration Programme is one of over 40 multilateral Technology Collaboration Programmes of the International Energy Agency. It was one of the first of such programmes, founded in 1977. Its current mission is to "advance international collaborative efforts for solar energy to reach the goal set in the vision of contributing 50% of the low temperature heating and cooling demand by 2030.". Its international solar collector statistics Solar Heat Worldwide serves as a reference document for governments, financial institutions, consulting firms and non-profit/non-governmental organizations.
Renewable thermal energy is the technology of gathering thermal energy from a renewable energy source for immediate use or for storage in a thermal battery for later use.
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