Solar Thermal

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Solar Thermal systems use Solar energy to provide heat (as opposed to Solar Photovoltaic systems which generate electricity).

There is a wide range of Solar Thermal technologies in use today. Solar Panels providing Domestic Hot Water are the best known, but many other hot water and other types of solar thermal systems also exist. A good site to get some idea of the range of options and their uses is Builditsolar.com


Hot Water

For DIY purposes in the UK the most popular application is hot water. Solar hot water can, if well designed, provide the majority of water heating for a house. To provide all water heating all year round would require greater solar system cost, giving a lower percentage payback per annum.

It is possible to use hot water panels with a thermal store to combine the output of solar panels with conventional sources (such as conventional boilers and/or electric backup heaters) and possibly other renewables (e.g. ground-source heat pumps and waste water heat recovery).

Such combined thermal store based systems can also contribute to space heating, via UFH and radiators, as well as DHW, but this is usually not done as such a setup gives lower performance and higher costs compared to other space heating options.


Collectors

There are 2 types of collectors in popular use in Britain, flat panels and evacuated tubes.

Flat Panels

Flat panel collectors typically deliver hot water in summer, and anything from nothing to warm water in winter. Their efficiency is high with cold water, but as water teperature rises their efficiency falls off badly, usually falling to around 0%.

Flat panels have lower installation cost than vacuum tubes, and relatively good performance under cloudy skies, as they are non-directional.

Flat panels deliver much better Return On Investment (ROI) when used to heat water to low temperatures, but for hot water they struggle to offer much output in winter.

2m^2 is a common flat panel collector size for domestic systems.

Selective coatings are common on flat panels to give less radiated heat loss than black paint. These coatings are generally based on metal oxides.

Vacuum Tubes

Evacuated tube collectors consist of 2 glass tubes, one inside the other, with vacuum insulation between the two. The inner surface of the outer tube is silvered on the underside, giving a moderate amount of direct sunlight concentration onto the inner water tube.

Vacuum tubes have much lower losses and higher efficiency than flat plates, and can heat water to much higher temperatures. However they do so at a much higher price, and are not issue free.

Tubes perform less well than flat plates under overcast skies, as they collect a lower percentage of incoming light, and under diffuse light this percentage drops even further. They also collect less energy than flat panels when heating cold water, when compared to either the same panel area or same collector cost.

Vacuum tubes are vulnerable to summer daytime power cuts. If coolant flow is lost for some time, the tubes reach a very high stagnation temp in direct sunlight. When power is restored, cold water is pumped into overheated tubes, and the glass tubes can break from the thermal shock.

When commissioning a vacuum tube system, power should be first applied when the sun is not up.


Combining Flat Plate & Tube

Each type of collector has its pros and cons, and each is better in some situations. Flat plate is better for warming low temperature water, so is the ideal choice for prewarming tanks, and for the low temperature end of a mixed panel system.

Vacuum tubes produce higher water temps, and do so with a longer heating season, so are ideal for the high temperature end of a mixed collector system.


Other Hot Water Collectors

There are many other types of collector besides the 2 most popualar types. Most of these were designed in attempts to obtain better performance and lower cost than the 2 popular ones. Many such collectors have a good amount of support among solar designers.


Frost protection

Water ruptures pipes due to freezing, and any solar hot water system must avoid this.

There are 4 ways to protect against freezing.

  1. Antifreeze in the solar collector circuit, plus a heat exchanger between this circuit and the heated water
  2. Draindown systems
  3. Drainback systems
  4. Freeze-safe collectors

Antifreeze is by far the most common method of frost protection. Non-toxic antifreeze must be used, not car antifreeze.

Draindown systems are manually drained when frost is likely in winter, and refilled when risk of frost has passed. This is a common low cost option for DIY systems. It is essential not to forget to drain the system, else the collector plumbing is likely to burst.

Drainback systems empty themselves by gravity every time the pump stops. This is an effective strategy, but requires that the panels be above the pump and the header tank of the water being heated.

Freeze-safe collectors are rarely seen.


Space heating

Solar space heating can deliver better payback than hot water systems. They have a much longer operating season than hot water heating systems due to different design and operating conditions, which result in much higher operating efficiency.

We can divide the approaches to solar space heating into 2 categories, simple systems that provide only part of a house's heat requirements, and more complex systems designed to provide all a house's heat, or the great majority of it.

Designing a system to provide all the heating a house needs is a complex exercise requiring some understanding of thermodynamics. It also requires storage, which is bulky and adds significantly to system cost. Design of such systems is a custom job for each house, and will for almost all people involve significant learning from experts such as at news:alt.solar.thermal.

Simple systems contributing only part of a building's heat requirement can be simple and low cost, and require much less technical knowledge. These typically have no heat storage. Effective systems can be made for a few hundred pounds using new materials, and provide daytime heating only. ROIs as high as 100% have been obtained with such systems in some cases.

Solar space heating can be combined with other heating technologies if desired to produce a complete whole house heating system.

Using hot water panels as the basis for space heating is not usually done due to the higher costs and poorer performance of water panels. However it is sometimes used as a means to enable heat storage. In these cases a large storage tank is used, and heat emitters using warm water are used, such as UFH.


Collectors

One design of collector is popular with warm air systems. This consists of a basic frame with a thin clear rigid plastic front. A second layer of glazing is sometimes used, with plastic film being a low cost option. There are air inlet holes at the bottom rear and outlet holes at the top rear of the panel, and the back of the panel is foam insulated. 2 layers of black mesh cloth are fitted at an angle inside, sloping from the back at the bottom to the front at the top.

Cool air (from the house) enters the panel's bottom holes. This air touches the front of the panel. As the air rises in the panel it travels through the holes of the 2 layers of black mesh. When at the top, the air is in contact with the rear insulation, and not the plastic front. The air now returns to the house through the top panel holes.

With non-thermostatic panels, a plastic film damper is fitted over the top return hole to block airflow after dark.


The Advantages of Warm Air

Efficiency

Warm air collectors are much more efficient than hot water panels, and retain this efficiency advantage through the day. This allows effective operation in much lower outdoor temps as well as much greater heat recovery.

Solar panel efficiency depends on a few factors, such as panel design, output temperature difference from ambient, and stagnation temperature. When output temp equals stagnation temp, this means that panel losses are equal to gains, and efficiency is 0%. Flat hot water panels normally run in this mode every day. Once the water reaches the panel's stagnation temp, it can not deliver any more heat. However warm air panels run at relatively low output temp, thus retaining most of their design efficiency all day. Operating efficiencies of around 90% are normal.

Potential Returns

The amount of energy used to heat water is quite limited for domestic uses, so the energy return for hot water systems is inevitably limited, and is in most cases below £100 per year. Much more energy is used for space heating, so greater returns are possible with a warm air system.

Cost

Warm air panels require no plumbing, no antifreeze, no heat exchanger, no pump, and no more control system than plastic flaps. They are usually constructed and fitted at ground level, which eliminates the costs and risks of roof access, and makes DIY construction practical for more people.


Season

Because warm air panels need to deliver much lower air temps to be effective, they will operate well outside of the season for hot water panels.


Payback

Because of all the above factors, costs are generally lower and returns higher than hot water systems, leading to much higher ROI.

Disadvantage of Warm Air

The drawback most often quoted of these systems is their appearance. Much larger collector areas are used than for hot water, and the collectors are generally placed on walls at ground level. Glass can be used, but plastic is more popular for cost reasons.

When necessary there are ways to reduce visual impact, such as fitting them to walls at first floor height, or even fitting to a fence or outbuilding and ducting the warm air. Naturally these options add to the work and/or cost.

Warm air panels are also less often fitted to roofs, or designed to be one with a new roof. These systems require forced air circulation, and can be used for summer cooling as well as heat.


Performance and Payback

Performance and payback are both key issues with solar thermal technology. There are designs in use that deliver very good results in both these respects, but there are also many more systems that fail to do so. Thorough evaluation of any proposed system is needed to be sure it will perform well and pay well.

Solarthermal is not an area where you can go to a random professional, purchase a system, and expect it to pay its way and perform satisfactorily. Some systems do achieve these goals, but a lot don't. Thorough evaluation is important.

Professionally installed systems come with much higher price tags than DIY units, and this makes achieving financially positive payback much more difficult.


Return on Investment

ROI, or Return On Investment, is the money a solar system saves in one year as a percentage of system cost. As an example, an £800 system saving £80 a year on heating costs would give 10% ROI, and this system would take 10 years to pay back its cost if interest were disregarded. IRL it would take a good deal longer due to interest.

Real world annual financial payback is ROI minus interest rate. Thus it is seen that greater than 10% ROI is normally needed to justify a solar system on financial grounds alone.

Solar sytems vary widely in performance, cost, payback and practicality. Systems with good ROI save you money, systems with poor ROI just cost money.

A Drain Heat Exchanger can reduce hot water requirements, thus reducing solar system cost.


Other Solarthermal Applications

  • Furnaces
  • Cooking
    • Ovens
    • Solar Hob
  • Solar Cooling


See Also

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