Solar hot water
Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and a reservoir or tank for heat storage and subsequent use.
Systems use sunlight to heat water. They may be used to heat domestic hot water, for space heating or to heat swimming pools. These systems are composed of solar thermal collectors, a storage tank and a circulation loop. The three basic classifications of solar water heaters are:
Batch systems which consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs, however; the exposed tank can be vulnerable to cool down.
Active systems which use pumps to circulate water or a heat transfer fluid.
Passive systems which circulate water or a heat transfer fluid by natural circulation. These are also called thermosiphon systems.
In order to heat water using solar energy, a collector is fastened to the roof of a building, or on a wall facing the sun. In some cases, the collector may be free-standing. The working fluid is either pumped (active system) or driven by natural convection (passive system) through it.
The collector could be made of a simple glass topped insulated box with a flat solar absorber made of sheet metal attached to copper pipes and painted black, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In some cases, before the solar energy is absorbed, a parabolic mirror is used to concentrate sunlight on the tube.
A simple water heating system would pump cold water out to a collector to be heated, the heated water flows back to a collection tank. This type of collector can provide enough hot water for an entire family.
Heat is stored in a hot water tank. The volume of this tank will be larger with solar heating systems in order to allow for bad weather, and because the optimum final temperature for the absorber is lower than a typical immersion or combustion heater.
The working fluid for the absorber may be the hot water from the tank, but more commonly (at least in pumped systems) is a separate loop of fluid containing anti-freeze and a corrosion inhibitor which delivers heat to the tank through a heat exchanger (a coil of copper tubing within the tank.)
If a hot-water central heating system is also present, then either the solar heat will be concentrated in a pre-heating tank that feeds into the tank heated by the central heating, or the solar heat exchanger will be lower in the tank than the hotter one. It is important to remember, however, that the main need for central heating,is at night when there is no sunlight and in winter when solar gain is lower. Therefore solar water heating for washing and bathing is often a better application than central heating because supply and demand are better matched.
The water from the collector can reach very high temperatures in good sunshine, or if the pump fails. Designs should allow for relief of pressure and excess heat through a heat dump.
Economics, Energy and System Costs
In sunny, warm locations, where freeze protection is not necessary, a batch type solar hot water heater can be extremely cost effective. However, in higher latitudes, there are often additional design requirements which add to system complexity and increase the amount of equipment required. This has the effect of increasing the initial cost (but not necessarily the life-cycle cost) of a solar hot water system, to a level much higher than a comparable hot water heater of the conventional type. When calculating the total cost to own and operate, a proper analysis will take into consideration that solar energy is free, thus greatly reducing the operating costs, whereas other energy sources, such as gas and electricity, can be quite expensive over time. Thus, when the initial costs of a solar system are properly financed and compared with energy costs, then, in many cases the total monthly cost of solar heat can be less than other more conventional types of hot water heaters.
A passive system also known as a monobloc system, a compact system consists of a tank for the heated water, a solar collector, and connecting pipes all pre-mounted in a frame. Based on the thermosiphon principle, the water flows upward when heated in the panel. When this water enters the tank (positioned higher than the solar panel), it expels some cold water from inside so that the heat transfer takes place without the need for a pump. A typical system for a four-person home in a sunny region consists of a tank of 150 to 300 liters and three to four square meters of solar collector panels.
Direct compact systems are not suitable for cold climates if they are made of metals. At night the remaining water can freeze and damage the panels, and the storage tank is exposed to the outdoor temperatures that will cause excessive heat losses on cold days. Some compact systems have a primary circuit. The primary circuit includes the collectors and the external part of the tank. Instead of water, a non-toxic antifreezing liquid is used. When this liquid is heated up, it flows to the external part of the tank and transfers the heat to the water placed inside. However, direct systems are slightly cheaper and more efficient.
A compact system can save up to 4.5 tonnes annually of gas emissions. In order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in six years, and achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems do not use any external source for power (as water flows due to thermosyphon principle).
Flat solar thermal collectors are usually used, but compact systems using vacuum tube collectors are available on the market. These generally give a higher heat yield per square meter and cost about the same as flat collector systems.
Solar heating thermal collectors
There are three main kinds of solar thermal collectors in common use: Formed Plastic Collectors, Flat Collectors, and Evacuated Tube Collectors.
Formed Plastic Collectors (such as polypropylene, EPDM or PET plastics) consist of tubes or formed panels through which water is circulated and heated by the sun’s radiation. Used for extending the swimming season in swimming pools. In some countries heating an open-air swimming pool with non-renewable energy sources is not allowed, and then these inexpensive systems offer a good solution. This panel is not suitable for year round uses like providing hot water for home use, primarily due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than the temperature of the fluid being heated.
A flat collector consists of a thin absorber sheet (usually copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass cover. Fluid is circulated through the tubing to remove the heat from the absorber and transport it to an insulated water tank, to a heat exchanger, or to some other device for using the heated fluid.
Instead of metal collectors, some new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers being flexible and therefore freeze-tolerant, they are able to use plain water in them instead of antifreeze, so that in s
ome cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one with extra heat exchangers.
Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). The tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it.
Two types of tube collectors are distinguished by their heat transfer method: the simplest pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube placed in each of the glass collector tubes. The second type uses a sealed heat pipe that contains a liquid that vapourizes as it is heated. The vapour rises to a heat-transfer bulb that is positioned outside the collector tube in a pipe through which a second heat transfer liquid (the water or antifreeze) is pumped. For both types, the heated liquid then circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square meter than flat panels. However, they are more expensive and fragile than flat panels.
Solar thermal cooling
Solar thermal cooling can be achieved via absorption cycles, dessicant cycles and solar-mechanical processes.
The absorption cycle solar cooling system works like a refrigerator — it uses hot water to compress a gas that, once expanded, will produce an endothermic reaction, which cools the air. The main problem currently is that the absorber machine works with liquid at 90 °C, a fairly high temperature to be reached with pumped solar panels with no auxiliary power supply.
The same pumped solar thermal installation can be used for producing hot water for the whole year. It can also be used for cooling in the summer and partially heating the building in winter.
The size and complexity of combi-systems, 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 combi-systems 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.
Solar combi-systems 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.