SolarAirTech

We spoke to Dave Goodwin, Technical Manager at SolarVenti UK, about solar water heating and he gave us a detailed account of the science, costs, effectiveness and benefits of installing a solar heating system in your home. 

Read on for our spotlight feature on Solar Water Heating.

Solar water heating systems use special solar panels, usually termed collectors, fitted to your roof (or in some cases to a south facing wall). These collect free radiant heat energy from the sun and use it to warm water which is subsequently transferred to and stored in a hot water cylinder.

There are two common types of solar water heating panels. There are evacuated tubes, which produce more hot water output, albeit at a higher purchase cost, and flat plate collectors. Either type can be fixed onto roof tiles, while some flat plate collectors can be directly integrated into the roof itself.

Most collectors require a mixture of antifreeze to prevent burst pipes during the colder winter months. This necessitates that some sort of heat exchange coil is required, usually a finned solar coil in a replacement hot water cylinder, in order to transfer the heat to a usable and clean source.

There are some collectors which are freeze tolerant that do not require the water to incorporate any anti-freeze. One such system is SolarTwin, which uses special silicon piping in the collector which expands during freezing. Such systems can have a direct hot water feed into your existing hot water cylinder.

In almost all cases a boiler or an immersion heater is required to provide hot water at times when output from a solar collector is low, and there is a need to bring the water up to a higher temperature.

Although most collectors use a mains powered electric pump to push the hot water to the transfer coil, there are a few, including SolarVenti, which are entirely mains powered free, and which use a solar powered pump. These systems are suitable for off grid properties. 

There are also a few systems that use no pump at all, but which depend on thermo-syphoning. The downside of this type of system is that it means that the collector needs to be below the level of the hot water cylinder, which generally rules out any roof mounting.  

Evacuated Tube Systems

These are generally noticeably (and sometimes considerably) more expensive than flat plate collectors, but the bonus for this increased cost is that they work more effectively during times of lower levels of sunshine. 
The best quality evacuated tube systems will even produce some hot water at a reasonable temperature on heavily overcast days. 

There are actually two types of evacuated tube system, these are generally termed Direct Flow Evacuated collectors and Heat Pipe Evacuated tube collectors. The Heat Pipe type dominates the market here in the UK whilst Direct Flow types are still used widely in no frost zones such as the coastal Mediterranean regions. 

Costs will undoubtedly come down over the next ten years as production ramps up, especially in the far east. The downside is that a lot of the far eastern manufactured tubes are of poor quality, and the promised longevity of 20 years plus is seldom realised. We have seen very expensive systems where evacuated tubes had started to fail at only two years old. 
 

Flat Plate Collectors

Flat plate collectors come in a myriad of variations, but the vast majority are essentially insulated metal boxes with glazed or polycarbonate covers and a dark-colored absorber plate inside, commonly with insulation behind it. Heat from the sun strikes the absorber plate and is transferred to a fluid that circulates through the collector in tubes or piping, commonly made of copper or plastic.

There are many variations in the configuration of the piping inside the collectors which suppliers claim to make theirs ‘the best’, yet the output performance between even the most expensive one and a home made collector is often hardly that significant. 

Do I need to replace my existing hot water cylinder? 

Most suppliers of solar hot water systems will tell you yes. They can then charge for the supply and installation of a replacement hot water cylinder containing a finned solar coil. This will significantly increase both the capital cost and the installation cost of your system. 

Providing your existing hot water cylinder is in reasonable condition, we - SolarVenti UK - will tell you no. 
We can supply and install a clever device from Willis Renewables called a SolaSyphon. This, for an outlay of just under £300 (plus about a half a day of a plumber’s time, as opposed to £700-£800 [or more] and a whole day of a plumber’s time), will efficiently transfer your solar hot water to your existing cylinder without effort. In fact, some establishments who have tested this device claim it is far more efficient than a finned solar coil, as it stratifies the hot water directly into the top of your existing cylinder. 

The SolaSyphon can be used on either vented or unvented hot water cylinders without problem. All that is required is sufficient space alongside your existing cylinder to fit a device about 6” in diameter and about 1 metre long. 
 

I have a combi boiler – can I still use a solar water collector? 

The answer, generally speaking, is no. The vast majority of Combi boilers are unsuitable, as to work with solar heated water they need to be capable of accepting a pre-heated water supply (95% of them cannot). 
However, we can now supply a clever valve system called a CombiSol. This, together with a pre-heater cylinder (we can supply suitable 100L or 150L cylinders), means that your combi boiler can work in conjunction with a solar water heating system.  

There is a fair bit of plumbing effort involved, but it means that you can reduce the amount of energy that your combi system uses in providing on-demand hot water.

What does an installed solar water heating system cost? 

A difficult question to answer simply, as properties have different layouts, different existing water systems, and, potentially, different collector mounting positions. So the answer, so to speak, is as long as a piece of string 
The only way to provide you with a fair cost estimate is to understand your property’s layout and existing set-up in some detail. 

There are some very fairly priced systems out there, and there are some horrendously priced systems out there. When it comes to solar water heating, the old Latin term ‘Caveat Emptor’ (buyer beware) applies very strongly. 
We have seen good basic flat plate systems fully installed and working for less than £3000, and seen similar systems where people have paid £8000+ for a very similar set-up. We have seen fairly decent quality evacuated tube systems fully installed and working for under £4500, and yet seen similar systems installed where the householder has forked out £15000+ or more! And there you were wondering where all the old double glazing salesmen had disappeared to!?

Remember that solar water heating will generally save you  up to a maximum of ~£150 per annum (depending on your property’s size, the number of occupants and the level of hot water usage, though for most properties it is considerably less than this. See the Energy Savings Trust assessment of annual savings. So try working our your return on investment 

Shop around, get several quotes, speak to existing customers, and check testimonials. All of these seem to be common sense, yet some people just don’t seem to bother. Oh, and don’t forget to work out the genuine ROI (return on investment) – any salesman that tells you your solar water system will pay for itself in under 10 years is very likely telling complete ‘porkies’.  
 

So why buy a SolarVenti hybrid system? 

There are two main reasons: 

1.You get a ‘two in one’ purchase – our systems provide both solar hot water AND ventilation, dehumidification and supplementary space heating, all out of a single unit.

2. Price. With our SV30H Plus, you get a solar ventilator, dehumidifier and supplementary space heater for ~£1400. And for another ~£1450 (probably cheaper than any comparable stand alone product), you also get a decent quality flat plate solar water collector too. (Prices excl. VAT) 

And a load of other reasons too:

1.Systems are simple and require little maintenance. With so few components, failure is a rarity.

2. Our systems are, in many cases, DIY installable

3. You can optimize the unit during running. We have customers who use only the solar water heating part of the system through the late spring,, summer, and early autumn. They then switch off the water system and use only the air heating component during the rest of the year. Other customers leave both system components running year round.

4.They can be used where no mains electricity is available.

5. Being entirely solar powered, they never cost anything to run.

6. They provide warmed air ventilation to your property too, meaning that moulds and mildews’, condensation etc. are virtually eliminated.

7. The drier air that they output makes your house far less hospitable to unwelcome guests such as house dust mite. So they are theoretically good for people who have asthma, or who suffer from allergies too.

8. The warmed dry air can actually reduce your winter heating bills and hence your overall energy consumption.