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

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 temperature rises their efficiency falls off severely, usually falling to around 0%.

Flat panels have much lower purchase cost than vacuum tubes, and fairly good performance in summer.

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 useful 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.

There are 2 possible methods of heat transfer with vacuum tubes. One type runs the water through the inner tube. Becoming more popular now are tubes using a volatile heat transfer liquid that carries the heat to a small heat exchanger at one end of the tube. This type results in a smaller working volume of water.

Vacuum tubes have much lower losses and higher efficiency than flat plates, and can heat water to much higher temperatures. They also produce heat in winter at times flat panels fail to deliver. However they do this at a higher price per energy returned.

Vacuum tubes can be 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. However this is rare in practice.

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 aren't 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.

Plate vs tube efficiency 2.gif

  • Eta = collector efficiency
  • Delta T = water output temp minus ambient temp

The diagram shows how flat plates produce more at low and medium differential output temps, while vacuum tubes produce more output at high water temps. What this means in practice is that:

  • Flat panels produce summer hot water for less cost than tubes
  • Flat panels work much less well in winter

Combining Flat Plate & Tube

Each type of collector has its pros and cons, and each is better in some situations. For large systems, such as when feeding a block of flats, there's advantage in combining both collector types.

Flat plate is better for warming low temperature water, delivering much more warm water per cost, 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 at higher cost, and do so with a longer heating season, so are used for the high temperature end of the system.

The setup is to run 2 separate water heating circuits, one using the vacuum tubes to heat hot water, and one using the flat plate to heat warm water. Cold mains water feeds the warm water tank, and warm water feeds the hot water tank.

Better than separate tanks, the 2 may be combined into one tank or cylinder, relying on stratification to produce the 2 separate zones. The advantage is more & quicker hot water production in summer, when the flat plate can produce hot water as well as the vacuum tube collector. One larger tank also has less surface area per volume.

Its possible to take this multilayer approach further, with a drain heat exchanger or hot harry type preheater feeding the flat plate tank.

Improving Flat Plate Collectors

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

Secondary Glazing

  \ \\
   \ \\
    \ \\
     \ \\

This reduces heat loss at higher water temperatures, giving a higher stagnation temperature, improving efficiency.

Double glazed collectors should be designed to either prevent or survive overheating. This is not difficult but needs to not be neglected.

External reflector

|:  <--- reflector on wall
  \ \
   \ \
    \ \  <--- panel on lower roof
     \ \

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't lie flush with the panel, so are only practical with low mounted panels, and panels where a roof apex meets a wall.

Oversize Cover

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

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.

Other Hot Water Collectors

There are many other types of collector besides the 2 most popular types. Most of these are 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
  • 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
  • There are lots more too

Frost protection

Water ruptures metal pipes when it freezes, and solar hot water systems 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.

Drainback is typically achieved by having an air gap between the return pipe (from the solar panel) and the tank it heats. When pumping stops, the return pipe empties itself, then gravity empties the feed pipe.

Freeze-safe collectors

Freeze-safe collectors are ocasionally seen. No panel freezing precautions are needed. Copper pipe should be avoided.

Direct and indirect systems

Direct systems take hot water from the HW cylinder, run it through the panel and return it to the cylinder. This approach suffers several problems:

  • Antifreeze can't be used, some other method must be employed to prevent freeze damage to panels and pipes
  • Scaling reduces collector efficiency
  • Coating of other muck in collector pipes over time also reduces collector efficiency

Indirect systems use a water circuit that doesn't mix with the tank water.

  • The HW cylinder has an extra heat exchanger.
    • Costs money
    • Sometimes a replacement HW cylinder is fitted
    • Also possible to use an external exchanger connected to HW cylinder's input and output pipes
  • Antifreeze is used in the solar circuit


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 DSHW kits, with their high price tag, aren't normally able to pay their way in money terms.

Commercial Properties

Solar water heating systems for commercial property have much more potential, as some commercial premises use a lot more water, so the potential payback is much bigger. Commercial SHW systems can return thousand of pounds savings per annum.


A blocks of flats has more saving potential than a single house. Owners can install solar heating to trim fuel bills, or occupiers can in some cases band together to install solar HW.

The main advantage is that a system for 10 flats doesn't need 10 tanks, 10 pumps and 10 installations, just one, making the ROI opportunity better. The same principle is true with a Drain heat exchanger.

There's also some advantage gained from the 10 users evening out hot water use.

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 sufficient 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.

A small amount of insulation is placed under the pipe. A black polythene or paint layer between base and pipes improves heat capture.

Collector performance is significantly improved by allowing a gap about 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 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). If its practical (its normally not), polythene can be arranged under the panel to catch any possible leakage and direct it somewhere harmless. 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. Its used when an invisible system is required. Due to low efficiency, relatively large collection area is needed.

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


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

Silvered mylar sheet can be used to make a reflective curtain which may be hung on fencing or an overhead metal rail. Rope or light chain along the bottom makes it stable in light wind. This is drawn back when the pool is used, bathers don't want to be dazzled and cooked.

The amount of heat capturable this way is limited by pool and reflector size, but its an easy and high ROI method of delivering some heat, with no run cost.

There's more than one grade of silverd mylar film. Mylar emergency blankets don't have high levels of reflection.

Plastic film curtains are vulnerable to wind, and means should be provided to wrap them up out of harm's way. Dividing them into more than one layer, one above the other, much reduces wind vulnerability.

Solid wall reflectors survive any weather, but aren't cheap to build and would often be in the way.

Other Collectors

Other types of pool collector are also used.

Glass panels

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

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 a tiny pool, and can be made by kids.

Output of hose & bottle collectors can be boosted by sticking black polythene to the rear of the bottles on the inside. This is generally done with high temperature grease.

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 expense, 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 (25-20 / 60-20).

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

Plumbing to minimise Pump Power

System run 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. Twice the pipe diameter means twice the pipe width, twice the depth and much less resistance. Hence the use of 1" or larger hose for pool collectors.

Pumping power consumption is also reduced by plumbing collectors as paralleled pipes rather than one long series arrangement. 2 collectors in parallel require much less pump power than plumbed in series. This is particularly significant when using very large collectors, such as tarmac collectors, which are best plumbed as several parallelled pipes.

Finally pumping power is reduced by keeping the collectors low. Having to pump the water up to a rooftop much increases the electricity use per given flow rate.

Minimising pump energy use

Energy use is power x time, so a good control system minimises run time and energy use.

Pump energy use is reduced by using a differential thermostat rather than a timer. The pump then only runs when collector temp is significantly above pool temp. This is particularly necessary for tarmac collectors, whose temp rise lags far behind insolation, and which fail to give any useful output on many days. And a pool thermostat stops the pump when pool temperature is satisfied.

Reducing power use

If you have an existing pump that's more powerful than needed and its wasting power, some speed and energy reduction can be had by reducing the voltage applied to the pump. This is done with a transformer or a series capacitor or inductor. See Droppers for details.

Flood Prevention

Some simple precautions can minimise the risk of flooding.

Plumbing connections should be robust. Jubilee hose clips are less likely to cause pipe failure than wire based clips, or tied wire.

Hosepipe should be replaced when the surface begins to crack up rather than waiting until it breaks. Pipe can be inspected twice annually, especially at joints & connectors.

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

Parallel plumbing makes it easy to shut off one collector and continue working with the rest if a failure should occur. This can be done with valves or G 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 minimise stratification.

Space heating

Solar space heating can deliver better payback than hot water systems. These have a longer operating season than hot water heating systems due to different design and operating conditions, which result in 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 or most of a house's heat.

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, as its not necessary to warm the building up early in the evening. ROIs as high as 100%pa have been obtained with such systems in some cases.

Solar space heating can be combined with other heating technologies 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 can be done 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.


Out         :
    |      ,:
    |     ' :   <------ Sun
    |    ,  :
    |   '   :
    |  ,    :
    | '     :
    |,      :
In  ________:

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 at the bottom. 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 heated 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 like these, a plastic film damper is fitted over the 2 holes to block airflow after dark.

Shadecloth v black can collectors

With the shadecloth collector, heated air never comes in contact with the glazing. This much reduces glazing heat loss. With cans, heated air is generated on both sides of the can. The hot air outside the can meets the glazing, and heat loss occurs.

Hot surfaces radiate heat away. Black drink cans simply radiate this back out (their silver interior doesn't radiate much). Shadecloth reradiates in both directions, and the multiple layers mean that a lot of the reradiation is blocked from exiting the collector.

The shade cloth collector allows free flow of air, so if mounted on a wall, no fan is needed. Parallel strings of cans arent quite as good in this respect, and a bit more fan power is needed when roof mounting.

Finally the cloth collector is far less work to make.

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 and 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't deliver any more heat. However warm air panels run at lower output temp, thus retaining more of their design efficiency all day. Operating efficiencies around 90% are attainable.

Potential Returns

The amount of energy used to heat water is limited for domestic uses, so the energy return for hot water systems is inevitably limited, in most cases below £100 per year. Far more energy is used for space heating, making greater returns possible.


Warm air panels require no plumbing, no antifreeze, no heat exchanger, no pump, and no more control system than plastic flaps. They're usually constructed and fitted at ground level, eliminating the costs and risks of roof access, and making DIY construction practical for more diyers.


Because warm air panels need to deliver much lower air temps to be effective, they 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 said of these systems is their appearance. Much larger collector areas are used than for hot water, and the collectors are generally placed on ground floor walls. Glass can be used, but plastic is more popular because of cost. 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 air. Naturally these options add to the work and 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 night time 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.

Solar thermal is not an area where you can go to a random professional, purchase a system, and 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 difficult.

Return on Investment


ROI, or Return On Investment, is the money a 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 greater than 10% ROI is normally needed to justify a solar system on financial grounds.

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

Many solarthermal systems never pay back their cost. 2 well worn approaches to maximising ROI are to use discarded parts at minimal cost, and to minimise pump power consumption.


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, and what the interest level is
  • e) which sources of power your house already has, and what cost they are
  • f) what other plant the solar equipment displaces, if any, 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