Solar Thermal

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caletorolic4 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

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.


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 severely, usually falling to around 0%.

Flat panels have lower purchase & installation costs than vacuum tubes, and relatively good performance under overcast skies, as they are non-directional.

Flat panels deliver much better ROI (Return On Investment) 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 a vacuum as 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 have significant disadvantages too.

Tubes perform less well than flat plates under overcast skies, as they collect a much lower percentage of incoming light, and under diffuse light this percentage drops even further. They collect much 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 can reach a very high stagnation temp in direct sunlight. When power is restored, cold water is pumped into overheated tubes, there is some risk of breakage from thermal shock.

When commissioning a vacuum tube system, power should be first applied when the sun is not up, or else the vacuum tubes kept covered until power is applied.

Comparing Flat Plate and Vacuum Tube

The diagram shows efficiency (y axis) versus water output temperature (x axis) for the 2 common types of collector. Values are not placed on the scales because the numbers will vary significantly in real life depending on collector designs, and also ambient temp has a sizeable effect on system performance. (Diagram needs marking with 'Output temp' and 'efficiency', if not with specific figures)


The diagram shows how flat plates produce more at low and medium water output temps, while tubes produce more output at high water temps.

Combining Flat Plate & Tube

Each type of collector has its pros and cons, and each is better in some situations. There are 2 ways to arrange a dual collector type system.

2 circuits

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.

Single Circuit

Where the additional complication of 2 circuits is not wanted, the collector types are simply plumbed in series. Water goes through flat plate first, then the vacuum tubes. This substantially reduces total collector cost for a given return, compared to all tube systems.

Improving Flat Plate Collectors

There are ways to improve the performance of flat plate collectors. Most in need of improvement is the low efficiency with hot water output, due to limited stagnation temp. Improvement here can thus yield a considerable performance boost.

Secondary Glazing

This reduces heat loss at higher water temperatures, giving a higher stagnation temperature, as needed for greater performance. Performance of double glazed collectors should be designed to either prevent or survive overheating. This is not difficult but should not be neglected.

External reflector

An external reflector increases the stagnation temp of the panel, thus greatly increasing its efficiency with hot water, as well as capturing more sunlight. External reflectors can not lie flush with the panel, so are only practical with low mounted panels, and panels where a roof apex meets a wall.

Oversize Cover

Fitting an oversize clear cover creates a simple warm air collector that holds heated air on the outside of the hot water panel. This reduces hot water panel heat losses, increasing stagnation temp and efficiency with hot water output.

  \ \ \
   \ \ \        <-- 2nd cover
    \ \ \
     \ \ \
      \_\ \
       \   \    <--  this area generates hot air
        \___\   <--  reducing HW panel losses

Other Hot Water Collectors

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

  • Batch heaters
    • collector and preheat tank in one
  • CH radiator preheat panels
    • low cost batch preheating
  • Hot Harry type preheaters
  • Pool heating pipes buried in path or drive
    • Invisible, very large area, low temp, high power output
  • Thomason trickle collectors
  • Heliostats
  • Hosepipe collectors
    • very low cost summer hot water panels
  • Trough concentrating collectors

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. Antifreeze requires a separate water circuit for the collectors with a heat exchanger.


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

Freeze-safe collectors are rarely seen. This may be due to durability questions with plastic collectors.


Payback is inherently limited in domestic solar hot water, because there is only so much water heating bill that can be replaced with solar heat. The key to effective payback is thus to keep system costs low while maintaining an effective energy harvest.

Commercial vs Domestic

Commercial solar water heating systems have much more potential, since commercial users of hot water often use a great deal more water, thus the payback potential is much bigger. Commercial SHW systems often return thousand of pounds savings per annum.

One area where commercial meets DIYer is with blocks of flats. Owners can install solar heating to cut thousands off fuel bills, or occupiers can in some cases band together to install solar heat.

A system for 10 flats doesn't need 10 tanks & pumps, making the ROI opportunity better. The same principle is true with a drain heat exchanger.

Pool Heating

Pool heating is an especially well suited application for solar power, because:

  • Very high power is required, so payback is large
  • Heat is not needed in the coldest months
  • Low water temp means even the lowest cost solar panels operate at high efficiency

Pool heating is a warm water application rather than hot water, and this makes solar pool heating behave differently to solar domestic hot water.

Insulate first

If you're supplying a lot of heat to a pool, it makes sense to insulate it to reduce heat demand. Solar blankets reduce pool heat loss. Kids etc must never be allowed to play in the pool with the blanket on.


For outdoor pools, the pool itself is the first solar collector. Heat collection can be maximised by choosing blue tiles rather than white, and using a solar blanket to reduce heat losses.

Hosepipe spiral

Hosepipe panels make low cost, medium efficiency, high power output collectors. They are the most popular collector for DIY pool heating.

Hosepipe is wound into a flat spiral and tied in place. Clear greenhouse polythene sheet is placed on top of the spiral.

Preferably a small amount of insulation is placed under the pipe. A black polythene or paint layer between base and pipes improves heat capture. Polythene under the pipe may be arranged to catch any possible leakage and direct it somewhere harmless.

Collector performance is significantly improved by allowing a gap the same width as the hosepipe between each turn of the spiral. This allows a greater amount of direct sunlight onto each pipe turn, and develops more heated air around the pipes, increasing gain and reducing pipe to panel air heat losses.

Hosepipes don't last forever, and spiral collectors should be located so that a split pipe would not cause a flood, and so a leak will be noticed long before any harm is done (or in extremis, the pool is drained). See #Pumping for more information.

Hosepipe collectors need to be emptied when frost approaches.

Tarmac Collector

Tarmac pavement and driveways can be used to heat a pool by burying pipe under the surface. This makes a low efficiency collector, and is not one of the cheapest options. It is used when an invisible system is required. Due to low efficiency, relatively large collection area is needed.

Plumbing the piping as several parallel runs reduces pumping requirement. It also enables isolation of one run in case it should ever be damaged, thus ensuring very long collector life.


Sometimes it is convenient to use an external reflector to direct additional sunlight into the pool. This has the advantage of not requiring any plumbing, pump or power, so setup cost can be low, and running cost is zero.

Silvered mylar sheet can be used to make a reflective curtain which may be hung on wood fencing. Rope or chain along the bottom makes it stable in light to medium wind. This is drawn back when the pool is used, as bathers will not want to be dazzled and heated by reflected sunlight.

The amount of heat capturable this way is limited by pool and reflector size, but it is an easy and high ROI method of delivering some heat, and costs nothing to run.

There is more than one grade of silverd mylar film. Not all have high levels of reflection.

Plastic film curtains are vulnerable to severe winds, and means should be provided to wrap them up out of harm's way. Solid wall reflectors survive any weather, but cost much more to construct.

Other Collectors

Other types of pool collector are also used.

Glass panels

Enclosed glass panels look nice. These are standard for professional installs, but the price tag is relatively high as large collector areas are required.

Hose & Bottle Collectors

Only occasionally used. This consists of a flexible hose threaded with lots of 2 litre soft drink bottles with their bases cut off. The bottles are pressed up against each other to form continuous glazing. Typically the pipe is snaked along flower beds. Such collectors might be sufficient for the smallest of pools.

Output of hose & bottle collectors can be boosted by sticking silvered mylar to the rear of the bottles. This is done with high temperature grease, with the mylar on the inside of the bottle, with the silver side of the film facing the grease. This acts as a non-ideal concentrating reflector, increasing the sunlight & skylight on the pipe. It is then necessary to use hot water rated pipe, as the pipe will see high temperatures. This also increases collector visibility.

Black polythene can be used instead for the back, this increases air temp in the bottles, thus heat capture.

Thomason trickle collectors

Also used for pool heating.


Optimum Pumping Rate

Pumping rate determines run cost and energy return. Too high a rate causes unnecessary runnning expenses, too low a rate and the energy harvest falls. Using unnecessarily high pumping power is a common mistake with pool heating.

Panel operating efficiency depends on, and can be partially determined by, water output temperature.

  • When output temp = collector stagnation temp, the panel is operating at 0% efficiency
  • When output temp = ambient air temp, the panel is operating at 100% of its maximum efficiency figure.
  • These 2 points are joined by a straight line graphically, so efficiency (as a percentage of max panel efficiency) is easily determined by measuring water output temp & stagnation temp.
  • As an example, A panel delivering 25C output in 20C ambient, and with stagnation temp = 60C, is running at 87% of its max efficiency.

Bear in mind that adding more collector area is generally a good deal cheaper than increasing pump power.

Pump Power Reduction

System running cost depends on pump power, as this is the one ongoing non-free energy input. Designing to minimise pump power can reduce run cost to trivial levels, whereas an inefficient design with a 500w pump can cost £100 a year to run. (example 10p/kWh, 8 hours a day pumping, 8 months per year use.)

Pumping power is much reduced by using wider bore pipe. Approx half the length of pipe twice the outside diameter will cover the same panel area. Thus doubling pipe width gives less than 1/4 the flow resistance for a given panel area. Hence 1" or larger hose is commonly used for pool collectors.

Pumping power consumption is also reduced by plumbing collectors as paralleled pipes rather than one long series arrangement. 2 spiral collectors in parallel require much less pump power than one continuous spiral twice the length. This is particuarly significant when using very large collectors, such as tarmac collectors, which should be plumbed as several paralleled pipes.

Finally pumping power is reduced by keeping the collectors low relative to the pool water level.

Flood Prevention

Some simple precautions can minimise the risk of flooding.

Plumbing connections should be robust. Hose connectors or jubilee clips are less likely to cause pipe failure than tied wire.

Hosepipe should be replaced when the surface begins to crack up rather than waiting until it breaks. Pipe can be inspected annually, paying attention to connection points, whch are often the weakest parts due to being under significant mechanical stress.

Polythene under panels can be used to direct drips or damage leaks somewhere harmless.

Parallel plumbing makes it easy to shut off one collector and continue working if a failure should occur. This is done with valves, or temporarily with clamps.

Minimising pump power minimises the volume of any leak that occurs.

Finally the pump inlet can be placed as high in the pool water as possible as a final defence against flood. This should not be used as the only defence when perishable collectors are used, such as hosepipe spirals. Every layer of defence adds more protection. When using this approach it is desirable to return warmed water to the bottom of the pool to avoid stratification.

Space heating

Solar space heating can deliver better payback than hot water systems. These 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

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. Daytime heat also reduces evening heating, since it is not necessary to warm the building up early in the evening. 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, to improve collector efficiency.


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


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.


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.


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.


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. Corrugated is cheaper than flat.

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 just assume it will pay its way and perform satisfactorily. 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.


EROI, or Energy Return On Investment, is the amount of energy a system harvests per year as a percentage of system construction and installation energy use. EROI is harder to calculate than ROI because methods of working out embodied energy vary, and what they include varies.

How much Payback?

ROI and EROI depend on several factors:

  • a) system design, which makes major differences
  • b) how you assess the embodied energy - and assessed figures vary widely
  • c) what you factor in, eg what your cost of labour is, which varies widely by location and by circumstance, and whether you treat it as a labour cost or hobby activity
  • d) Whether interest on capital is taken into account
  • e) which sources of power your house already has, and what prices they are
  • f) what other plant the solar equipment displaces, and the avoided cost of installing it
  • g) whether the design incurs ongoing maintenance costs, and if so how these are costed
  • h) system lifetime
  • i) system reliability, any shortcoming in which can incur significant additional costs.

In short, system payback varies from >100% pa to never in the life of the universe, and needs to be calculated on a per case basis.

Other Solarthermal Applications

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

See Also