Make the most of so­lar ther­mal

Nigel Grif­fiths re­veals what you need to know to op­ti­mise the per­for­mance of your so­lar ther­mal setup and max­imise sav­ings

Build It - - EDITOR’S LETTER -

Ex­pert ad­vice on what you need to know to get the best per­for­mance from your so­lar water heat­ing sys­tem

So­lar ther­mal sys­tems are in­stinc­tively at­trac­tive to those look­ing to cre­ate a more sus­tain­able home with lower bills. This is sim­ple, low tech that en­ables you to use heat from the sun to do what would oth­er­wise take fos­sil fu­els to achieve – and some pan­els are man­u­fac­tured right here in the UK.

A well-de­signed and op­er­ated so­lar ther­mal ar­ray can pro­vide up to 50% of a house­hold’s do­mes­tic hot water. Get it wrong, how­ever, and your pan­els could be just an ex­pen­sive bit of eco-bling. This ar­ti­cle ex­plores the crit­i­cal fac­tors that will en­able you to max­imise so­lar ther­mal yield.

How so­lar ther­mal sys­tems work

Be­fore we get into the tech­ni­cal­i­ties, let’s sum­marise how the setup works (skip this bit if you’re fully up to speed).

So­lar water heaters col­lect en­ergy from the sun (and day­light) and use this to raise the tem­per­a­ture of the water we con­sume for wash­ing and bathing. Much of the en­ergy re­quired to heat water is used to take it from cold to luke­warm tem­per­a­ture. This means that, even on days with dull weather, the pan­els can still do some work.

The to­tal amount of hot water pro­vided by so­lar ther­mal pan­els is known as the so­lar frac­tion. A sec­ondary source of heat (usu­ally a fos­sil fuel boiler) will be re­quired to do the work dur­ing the win­ter and as backup at all other times.

The key com­po­nents that make up a so­lar water heater are the col­lec­tors, a twin-coil cylin­der and a con­troller. The vast ma­jor­ity of these sys­tems are in­di­rect. This means the liq­uid that cir­cu­lates through­out the col­lec­tors is held in a sealed loop, and the warmth is trans­ferred to the hot water cylin­der by means of a heat ex­changer (a coil). In this kind of setup, the ves­sel uses two sources of heat – the so­lar coil and the boiler coil – hence it is known as a twin-coil cylin­der. Cold mains water en­ters at the bot­tom of the unit, is warmed by the sun and then ex­its at the top in or­der to feed your baths, show­ers and basins.

As heat rises, the lower sec­tion of the cylin­der is nor­mally cooler than the top. The so­lar coil is there­fore lo­cated at the base where the water is coolest, to make the best use of what­ever so­lar re­source is avail­able.

El­i­gi­ble in­stal­la­tions can ac­cess quar­terly cash­back pay­ments for seven years via the Re­new­able Heat In­cen­tive (RHI), which cur­rently pays out 20.66 pence per kwh of en­ergy gen­er­ated. For de­tails of how the scheme works visit

Max­imis­ing so­lar yield

For any self builder or ren­o­va­tor, value for money is a ma­jor con­sid­er­a­tion. Here are the key fac­tors to bear in mind to en­sure you get the most of your so­lar hot water sys­tem.

1 Panel ori­en­ta­tion

Ideally, the ther­mal col­lec­tors should be mounted fac­ing south (any ori­en­ta­tion south-east to south-west will still work). The best an­gle in the UK is around 35° to the hor­i­zon­tal. This is con­sis­tent with many of our pitched roofs, although col­lec­tors can be in­stalled on flat roofs or even on the ground with a suit­able mount­ing bracket.

The trick is that the site should be fully ac­ces­si­ble to the sun’s en­ergy. You need to avoid any over­shad­ing from ad­ja­cent build­ings or trees, as it will sig­nif­i­cantly re­duce

the so­lar yield. Try to con­sider what may hap­pen fur­ther down the line, too. For in­stance, if you’re look­ing at putting pan­els on a sin­gle storey, is there any risk that they could be­come over­shad­owed by grow­ing trees out­side of your con­trol, or your own or a neigh­bour’s fu­ture ex­ten­sion?

2 Flat plate vs evac­u­ated tube

Two main types of so­lar ther­mal col­lec­tor are in com­mon use. Flat plate ver­sions nor­mally have a Per­spex cover, mounted over an ab­sorp­tive panel through which the col­lec­tion fluid is pumped. The whole unit is in­su­lated be­low and at the sides to re­tain the warmth.

Evac­u­ated tube col­lec­tors com­prise one tube set in­side an­other, with a vac­uum be­tween the two. The vac­uum al­lows ra­di­a­tion from the sun to pass to the in­ner tube (con­tain­ing the col­lec­tion fluid) while re­duc­ing heat loss.

In the UK cli­mate, evac­u­ated tube sys­tems will be more ef­fi­cient than flat plate col­lec­tors – but they tend to be more ex­pen­sive to in­stall for the same yield. So good site­spe­cific de­sign is needed to de­cide be­tween the two.

3 Bal­anc­ing the sys­tem

The other ma­jor de­sign con­straint to be aware of is the bal­ance be­tween the out­put of the col­lec­tors and the size of the hot water stor­age cylin­der. Too small a cylin­der (or too large a col­lec­tor for a given tank size) will mean you can’t store all the heat gen­er­ated – so it’s sim­ply wasted.

Con­versely, if the col­lec­tor is too small for the tank size, out­side of sum­mer­time, the pan­els will not give suf­fi­cient en­ergy to get the water up to the cor­rect tem­per­a­ture. So it will need to be sup­ple­mented by a backup heat source more of­ten than would oth­er­wise be the case.

Your cho­sen in­staller is re­spon­si­ble for get­ting this bal­ance right, so it’s noth­ing you’ll need to worry about. But it’s an ex­am­ple of mak­ing sure you hire the right pro and take ref­er­ences to get the most out of your sys­tem.

4 The so­lar con­troller

This is the core of your ther­mal ar­ray. It senses when the water in the stor­age cylin­der is a few de­grees cooler than the col­lec­tor pan­els and switches on the pump in the so­lar loop to trans­fer that heat from the col­lec­tors into the tank.

As it takes en­ergy to run the pump, there needs to be a gap be­tween the tem­per­a­ture at the col­lec­tors and in the heat ex­changer to com­pen­sate for these par­a­sitic losses.

The most com­mon backup source of heat to sup­ply a twin coil cylin­der is a fos­sil fuel boiler run­ning off nat­u­ral gas, LPG or oil. Oc­ca­sion­ally this might be a biomass boiler or a heat pump (although the lat­ter is not quite as ef­fi­cient in get­ting water up to tem­per­a­tures as high as 60°C).

Many twin-coil cylin­ders have a sec­ondary backup in the form of an im­mer­sion. An elec­tri­cal re­sis­tance heater is the most ex­pen­sive and car­bon-in­ten­sive form of hot water pro­vi­sion – so it pays to min­imise its use. In many sys­tems it is only switched on to help pre­vent le­gionella by rais­ing the tem­per­a­ture to a higher than nor­mal level.

As a do­mes­tic hot water cylin­der is ef­fec­tively strat­i­fied, with cold water en­ter­ing at the bot­tom and hot ex­it­ing the top, the backup boiler will nor­mally only be used to heat the up­per half of the tank. This leaves the bot­tom sec­tion cooler and in­creases the like­li­hood of a tem­per­a­ture dif­fer­en­tial be­tween col­lec­tors and heat ex­changer in the tank, thus in­creas­ing the ef­fec­tive­ness of the sys­tem.

There is an in­her­ent ten­sion be­tween this tem­per­a­ture dif­fer­en­tial and the flow rate on the so­lar loop. A faster flow rate will trans­fer heat to the cylin­der more quickly, but it means the pump uses more en­ergy. A lower flow rate can be achieved if the so­lar heat ex­changer in the cylin­der is larger – but this will in­crease the cost of the sys­tem.

Your in­staller will nor­mally set the flow rate on the con­troller to op­ti­mise the sys­tem – although some units can ad­just this in­tel­li­gently to max­imise per­for­mance.

5 How you use your do­mes­tic hot water

If you want to get best per­for­mance from your so­lar ther­mal, you need to use the en­ergy when it’s avail­able. I don’t mean to the ex­treme of tak­ing baths in sum­mer and not dur­ing win­ter – but if you can use stored hot water in the morn­ings, then the tank will have a chance to re­heat dur­ing the day when the sun’s shin­ing.

The same prin­ci­ple is true of ap­pli­ances. Many wash­ing ma­chines only have a cold water feed and there­fore rely on an in­ter­nal elec­tri­cal re­sis­tance heater to get the water to the re­quired tem­per­a­ture. How­ever, if you have both hot and cold sup­plies to your wash­ing ma­chine and use it dur­ing the day­time, then for much of the year you will be re­duc­ing your en­ergy con­sump­tion.

6 Don’t for­get main­te­nance

So­lar ther­mal set­ups are fairly straight­for­ward, so re­quire rel­a­tively lit­tle main­te­nance – but pe­ri­odic up­keep is still im­por­tant, as for all heat­ing sys­tems. To en­sure op­ti­mal per­for­mance, reg­u­larly check the ar­range­ment is cor­rectly pres­surised, con­tains the cor­rect fluid com­po­si­tion and that the col­lec­tors are free of dirt and de­bris.

Above left: A two col­lec­tor, in-roof flat plate sys­tem from Grant So­lar Ther­mal Left: This di­a­gram shows how warmth cir­cu­lates from two heat sources into a twin-coil cylin­der, be­fore be­ing ab­sorbed into do­mes­tic hot water and di­rected to the re­quired out­let

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