Solar Heating Article

7/29/2019 Solar Heating Article

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Demystifying Solar Heatingby Andy Wilson

The sun is the source of most of ourrenewable energy supplies. Without

the sun, there would be no weather orclimate, no rivers, lakes or wind. Weunconsciously use solar energy everysingle day in a myriad of ways, from thefood we eat to the colours we enjoy in

the world around us.

Essentially, the sun is an enormousfusion reactor which radiates energyout into space. This radiated energy isspread over a wide spectrum of wave-lengths, ranging from ultraviolet toinfrared with the visible part of the

spectrum (which gives us colour) in themiddle.

It is the infrared radiation which hasthe ability to warm surfaces it comesin contact with. The darker and lessreective the surface, the more heatenergy can be absorbed. On a globallevel, the amount of solar radiation be-ing reected back into space is chang-ing as a result of melting sea ice in

Arctic areas. The exposed sea is much

darker and less reective than the icewhich previously covered it, resultingin more solar radiation being absorbed.

As is now known, this has serious im-plications for the stability of the earthsclimate.

On a much smaller scale, solar radia-tion can be captured by solar panelsto provide useful heat for domesticor municipal use. At these northernlatitudes, solar heating systems arepredominantly used for heating water,

rather than for space heating.

This is because 77-81 percent of allsolar radiation reaching Ireland arrives

between the beginning of April and theend of September, when space heatingrequirements are very low. Domestichot water heating requirements on theother hand, may actually be higher inthe summer months because peopletend to take more showers when the

weather is warm.

A Year of Extremes

Because the sun is much lower in thesky in winter, the difference in solar ra-diation reaching Ireland on an averageday in December and an average day inMay, June or July is roughly a factor often. These average gures, however, donot tell the full story:

An analysis of twelve months solar datafrom the Met Eireann station at Dublinairport revealed that almost 100 times

more solar radiation reached Dublin onthe best day of the year ( 7.99 kWh/m2)than on the worst (0. 08 kWh/m2).

Of the 97 days when the total solardaily solar radiation at Dublin airport

was under 1kWh/m2, only 1 occurredbetween the beginning of April and theend of August while 82 occurred be-tween the beginning of November and

the end of February and included everysingle day in December. (see g 1)

Domestic Hot Water

Estimates of domestic hot water re-quirements vary widely but a ballparkgure for a family of four is 3500kWhof heat energy per annum or about10kWh per day.

Winter space heating requirementson the other hand may be as high as

200kWh in a single day. The fam-ily which uses 2000 litres of oil overa winter (not unheard of) will easilyreach this gure on a cold windy dayaround Christmas time.

In Ireland it currently costs 5-8000 tohave a domestic solar heating systeminstalled. This seems extraordinarily

bad value for money. At current oilprices and typically boiler efciencies,it will cost under 400 per annum toprovide domestic hot water for a family

of four. Of this, the solar fraction (theproportion of total hot water provided

by the solar heating system) may be aslittle as 15-20%.

In such cases, the value of the solar hoteditors note: kWh/m2 =kilowatt-hours ofenergy per square meter of horizontal surface

Sunny Days

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photo bigstockphoto.com


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g 1 At northern latitudes there is 10 times more solar radiation in June than in December

g 2 At collector areas greater than 6m2, the solar

fraction increases only marginally.

Volker Quaschning: Renewable Energy

water may be no more than 60 perannum. A detailed assessment of solar

water heating by Seamus Hoyne onbehalf of the Tipperary Institute esti-mated the payback on a solar thermalsystem as 40 years! (see www.ilsu.ie/documents/SemRE/SHSolar.pdf)

Winter Contribution Tiny

Although it is often claimed that agood solar heating system will provideup to 20 percent of wintertime domes-tic hot water requirements, this is notthe case.

The extremely low levels of solarradiation from mid November to earlyFebruary will mean almost total de-pendency on conventional methods ofheating water.

Even on those lovely but rare sunnywinters days when there is potentiallygood solar energy to be harvested,no hot water can be drawn off a solarcollector until it is heated to a highertempature than the hot water cylinderto which the collector is connected.

In general, it can be assumed that on90-110 days per annum, the outputfrom the average solar heating system

will be close to zero.

For a household which has a daily hotwater requirement of 10kWh, a 6m2solar collector with an overall systemefciency of 40% (well above average)

will fully meet their needs only ondays on which the total solar radiationexceeds 3kWh/m2. 1

An analysis of gures taken over atwelve month period by Met Eireannat Dublin airport showed that solar

radiation exceeded this level on a totalof 124 days, but that only one of thesedays occurred between Sept 21st andMarch 21st. On the best days in May,June and July however, solar radiationexceeded 7kWh/m2.

Storage Capacity

The size of the cylinder used to storethe hot water is also a major factor indetermining the efciency of a solarheating system. Many cylinders holdonly about twenty four hours supply ofhot water, so a number of sunny daysin a row might count for very little ifthey are followed by two really poordays of wet and cloudy weather.

To best utilise Irelands very variablesolar resource, where the daily solarradiation on two consecutive days may

vary by a factor of seven (as occurred25-26 June 2006), a minimum of twodays storage capacity is essential.

The performance of a solar heatingsystem can be badly compromised bypoor lagging of the hot water cylinderand pipes. Unlike an oil red boiler, so-lar collectors can not be turned up inorder to compensate for lazy or carelessinstallation practices. Whereas a 25kW

boiler will eventually overcome circula-tion heat losses, the solar collector may

simply be rendered redundant.

Resource Management

A consideration only rarely discussedin the solar energy sector is the man-agement of the resource. To give anexample, a very large percentage of newhouses are tted with electric showers

which heat their own water using aninternal heating element. Many dish-

washers and washing machines alsoheat their own water, though some canaccept external sources of warm waterand will then boost the water to thedesired operating temperature using aninternal heating element.

In many cases, half to two thirds of amodern households domestic hot waterneeds will be accounted for by appli-ances which are unable to utilise solarheated water. At best about 60 percentof the remaining hot water require-ments will be met by a solar heatingsystem, and most of this contribu-tion will occur during in the summermonths.

This suggests that less than one third ofthe typical households total hot waterrequirement can be met by a solarheating system, and in some cases thegure could be as little as one sixth.Doubling the size of the collector makes

very little difference (see g 2).

These gures are considerably at oddswith the 60 percent of all domestic hotwater requirements mantra repeatedendlessly by the solar heating indus-try and even by SEI, the Government

body responsible for implementing thegrants for renewable energy.

The way in which the solar resource is

used on a day to day basis will also be asignicant factor in the efciency of thesystem. For example, it is better to usethe hot water as soon as possible afterit is heated, for every hour it sits in thecylinder it will be gradually cooling.

Where there are non-electric showersfed directly by the hot water cylinder, itis better to take showers in the eveningrather than rst thing in the morning.This is particularly the case duringMarch or October when solar output

can still be signicant but may be com-promised by the use of conventionalenergy sources to heat the thermalstore. Unfortunately, very little attempthas been made by either the State or

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Fig 3 Most locations in Ireland and Britain receive between

800 and 1000 kWh/m2 of solar radiation per annum

the solar energy industry to educate theconsumer in solar energy management.

Instead, the unspoken assumption isthat scalding hot water must always

be available on demand, with nary athought about where the non-solarfraction of domestic requirements willcome from.

Space Heating Myths

It is stated on the SEI website that asolar thermal system providing spaceand water heating can cover 30 to 40percent of the annual [total] heating re-quirement of a house in Northern Eu-rope. Elsewhere on the site, however,there is government data indicatingthat the average house in Ireland re-quires 20,000 kWh of heat energy perannum for space heating. Unfortunate-

ly, there is something of a contradictionin the two pieces of information.

Assuming four fths of the annualspace heating demand occurs betweenthe beginning of October and the endof March, and that a solar collector isable to capture 40 percent of incomingsolar radiation in the winter months(very optimistic), 40m2 of solar panelsand a thermal store in excess of 20,000litres in size would be required in orderto meet 30-40 percent of heating

requirements from solar. This wouldseem a trie ambitious for the averagehousehold.The bottom line is that solar collectorsare very good for heating water from

April through to September, and canmake some useful contribution in thetwo months either side. For the simplereason that the time of year when theiroutput is lowest coincides with theperiod when domestic space heatingrequirements are highest, they makeextremely poor space heaters.

Evacuated Tube or Flat Plate

It is often taken as a given that evacu-ated tube collectors are more efcientthan at plate collectors. This howevermay not be the case. Tests carried outon 160+ different solar panels by theinternationally acclaimed research

organisation Solartechnik PrufungForschung (SPF) found that the grossefciency of at plate collectors wasconsiderably higher than evacuatedtube collectors.

The gross efciency is a measure of thepercentage of the total solar radiationfalling on the solar collector which thecollector is able to capture. The averagegross efciency of the 120 at platecollectors tested by SPF was about 70percent, whereas the 42 evacuated tube

collectors examined only averaged 50percent.2

Massaging the Data

The manufacturers of evacuated tubesare able to claim their products aremore efcient than at plates by thesimple expedient of comparing only

the area of the aperture (the bit thesun shines through) of the two types ofcollector. On average, the aperture ofan evacuated tube captures solar radia-tion more efciently than the at plate.There is however considerable varia-tion between different models.

Much more signicant however is therelationship between the aperture areaand the gross dimensions of the panel.

While the aperture of a at plate collec-tor may exceed nine tenths of the total

area of the collector, the proportion isis lttle more than half with evacuatedtube collectors. Any solar radiation fall-ing on the space between the evacuatedtubes is effectively lost.

The SPF results also gave some clearindications as to the source countries ofthe best and worst performing panels.Of the at plate collectors tested, mod-els manufactured in Austria and Ger-many were among the most efcient.The best performing evacuated tube

collectors were manufactured inSwitzerland and Northern Ireland.3At the other end of the scale, six of theten worst performing evacuated tubecollectors were manufactured in China.The tests showed that the best perform-ing collectors were more than twice asefcient as the worst ones.

Working Efciency

It should be noted that the workingefciency of a solar collector when

used for domestic water heating isonly about three quarters of the grossefciency. This is because the efciencyof the collector varies according tothe manner in which the solar-heated

water is used. Efciency tends to fall asthe temperature of the thermal storeincreases. In domestic situations, therelatively high temperature of the ther-mal store means that solar panels willoperate at reduced levels of efciency.

When calculating likely output from a

solar heating system, it is also neces-sary to factor in heat losses from thethermal store and circulation system.Thus a 40 perecent working efciencymay translate into a 30 percent overallsystem efciency. Of the 900 kWh of

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Fig 4 Where the hot water cylinder is higher than

the solar collector a pump may not be neccessary.

The water circulates by thermosyphon.

Fig 5 Where the hot water cylinder is lower or at

the same height as the collector, a pump is nec-

cessary for circulation.

solar radiation falling on each squaremeter of solar collector per annum,only about 270kWh will be utilised.

Are Swiss Results Valid?

It has been argued that because theSPF tests were carried out in Switzer-land, the results are not valid for coun-

tries like Ireland and Britain. Certainlythere are differences in the climates.Switzerland receives roughly one thirdmore solar radiation than Ireland an-nually and two to three times as muchduring December and January. Duringthe summer however the difference

between the two countries is small.Switzerland suffers much colder wintertemperatures which one would haveexpected to disadvantage at plate col-lectors as they tend to lose heat faster.However, this disadvantage may be

cancelled out by the lower wind speedsin Switzerland.

In Ireland there are more days withvery poor levels of solar radiation, andon these particular days the evacuatedtubes may out-perform the at plates.

As analysis of the Dublin airport guresrevealed however that the combinedsolar radiation on the 100 worstdaysof the year contributes only a little over6 percent of the total annual solar ra-

diation, astonishing as this might seem.So even if the evacuated tubes were toperform twice as well on these days, thenet gain would be very small.

UK Tests

In different tests, this time carriedout by the UK Department Trade and

Industry on 8 models of solar panels,the evacuated tube collectors used per-formed better than the at plates butthe small sample suggests these resultsshould be viewed with some caution.

The two evacuated tube collectorstested are among the best of their typeon the market and were certainly notrepresentative of the whole sector.The at plate collectors included vemodels glazed with synthetic materialssuch as twin wall polycarbonate, which

have much lower optical efciency thanglass.

One of the models tested was the SolarTwin, which uses a tiny solar photo-

voltaic panel to operate the circulationpump. The pump comes on wheneversolar radiation levels are sufcient todeliver the required electrical outputfrom the photovoltaic panel. While thisconveniently removes the need for so-phisticated controls, it also means thatsometimes the water is being circulated

when the collector temperature is lowerthan the temperature of the thermalstore. As a result the overall efciencyof the Solar Twin is low.4

While clearly there is a need for consid-erably more research into the perfor-mance of solar collectors, claims thatevacuated tubes collectors are more ef-cient than at plates should certainly

be treated with suspicion and need tobe properly qualied.

Flat Plates are the Future

The at plate collectors have a num-ber of other advantages over evacu-ated tubes. They require only half theembodied energy of evacuated tubes tomanufacture, even excluding the en-ergy used in transport to import evacu-ated tubes from places like China. Theproduction process associated with atplates is less complicated, which lendsitself to localised small scale produc-tion. Flat plates are normally cheaper

than evacuated tubes.

The tendency in most European coun-tries is towards at plates. Accordingto the European Solar Thermal Indus-try, of the 1,500,000 m2 of solar collec-

tors installed in 2005, over 91 percentwere at plates.

In Austria, many at plate collectorsare available in kit form, and there isa thriving industry based on low costDIY solar. At the end of 2005, Austriahad over 100 times more installed solarpanels per head of population thanIreland. Less than 1 percent of solarcollectors installed in Austria in 2005

were evacuated tubes, compared withan estimated 30-40 percent in Irelandand Britain.

The European leader in solar thermalis now Denmark, with over 7,000,000m2 of installed solar capacity. Ire-lands installed solar capacity is around20,000 m2, while Britains capacity isabout 240,000m2. As Denmark shows,installed capacity has little to do withclimate or annual solar radiation levels,

but is more a reection of nationalpriorities.

Price may also be a factor. In Ireland,domestic solar thermal installationsare overpriced by 2-300 percent. Byfar the most economical way to installsolar collectors is to forget the govern-ment grants which are available, buythe collector and hot water cylinderseparately and do the plumbing work

yourself ( with help from friends or alocal plumber if necessary).

Conclusions

Solar energy is an amazing resource,and the equipment to harness it forheating purposes, once installed,requires little maintenance and has along life. Unfortunately, Ireland, likeBritain, has got off to a very bad start indeveloping the sector. For a role model,

we should look to Austria, where al-most every town of any size has classesin DIY solar installation and where thecomponent parts can be purchasedlocally.

Fig 6 In northern Europe, less than half ofsolar radiation comes from direct sunshine

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The installation of any solar equipmentshould always be preceded by a carefulevaluation of energy demand, with theaim of streamlining energy usage andmaximising the energy efciency.For renewable energy to be used to

best advantage, education is probablythe single most important tool. Solarthermal technology is not rocket sci-ence. It could be taught in schools andat evening classes aimed at adults. Thelong term objective should be an indig-enous, localised solar thermal industryproducing low cost easy to install solarcollectors for mass deployment in amanner similar to Austria. There issome way to go.

Solar Heating GlossaryCollectorThe collector is the part which captures the solar radiation. There are two main types:

The fat plate collector, which can be thought of as a large radiator in a glass (occa-sionally polycarbonate or acrylic) fronted box.

The evacuated tube collector, which is more like a series of long transparent thermosasks each with a water pipe running up the middle.

Both come in a range of sizes, but 2-3m2 is fairly typical. Where larger installations areneeded, collectors are connected together in series or parallel.

Thermal StoreThis is just another name for the hot water cylinder. In spite of the solar industry pref-erence for expensive stainless steel cylinders, there are many good reasons for using themore traditional and much cheaper copper cylinder. Usually the cylinder will requiretwo internal coils, one for the solar circuit ( the lower coil) and one for a conventionalheat source such as a stove or boiler.

Circulation SystemThe circulation system is the bit which connects collector with thermal store. Usually itis a closed loop with integral expansion cylinder, pressure release valve or vent pipe to

relieve excess pressure should it occur. The longer the circuit, the greater the heat losseson route.

ControlsThe simplest solar heating circuits require no pump. Instead circulation is effectedthrough thermosyphon. For this to work, the collector must be below the thermal store,and there should be a constant upward gradient between the two. The beauty of this typeof circulation system is that nothing can go wrong. See Fig 4

In most installations, the collector will end up higher than the store, so a pump must beused for circulation. Ideally the pump should only be running when the collector tem-perature is higher than the store temperature, or there is the risk of re-circulating warm

water from the store back into the collector. See Fig 5

One solution is to use a controller which has two sensors to monitor collector and store

temperatures. When the collector is warmer than the store by a predetermined numberof degrees (usually 5-8) the pump is activated. When the difference between the twodrops, the pump turns off again.

The Solar Twin System, is to use a small solar PV panel to run the pump. The underlyingprincple is that if there is sufcient solar radiation to activate the pump, there is a needfor the pump to be going. It sort of works.

UserThe success of the system does require some intelligence on the part of the user. A solarpanel is not an immersion heater. It will not heat water on demand, but only when thereis an adequate level of solar radiation. Watch the system and see how it works. A usefuleducational aid is a small dial thermometer which can be clipped to the domestic hot

water supply pipe where it leaves the cylinder. This will indicate the water temperature atthe top of the cylinder (some solar pump controllers have digital temperature displays).

Rules of Thumb for Domestic Water Heating

The area of collector required is 1-2m2 per permanent inhabitant of the building. The lowend of the scale is more applicable for larger households as there will be some economiesof scale and greater degree of efciency within the system.

The volume of thermal store should be 60-120 litres per m2 of collector. The storeshould be much taller than it is wide, in order to facilitate stratication. A height to

width ratio of 2.5-3:1 is recommended. Stratication allows the hottest water to rise andform a layer at the top of the cylinder, where it can easily be drawn off.

The collector should be orientated somewhere between SE and SW and be in an openunshaded position. The angle of inclination is not critical but 20-40 degrees from thehorizontal will give best results. Only 40 percent of the solar radiation reaching us isfrom direct sunlight, the rest is defuse and comes from a much wider part of the sky.

On average, around 65-70% of the total daily incoming solar radiation occurs during a 6hour period straddling the middle of the day.

Notes

1 Assumes overall system efciency of 41.7%,

which is generous

2 The SPF website: www.solarenergy.ch is well

worth a visit. Click on the English language ver-

sion, go to test reports then collectors. There is

a downloadable pdf for each collector tested. On

page two of the pdf the gross efciency of the

collector is given. A gure of 0.408 for example,

means the gross efciency of the collector is

40.8%. The outputs in kWh/m2/yr are given for

aperture taken in isolation, so a further calcula-

tion is needed to determine the output per

square meter of panel.

3 Thermomax Ltd, Bangor

4 See www.dti.gov.uk/les/le16826.pdf.

Forthcoming Tests

The Sustainability Institute intends tocarry out tests on a number of commercialsolar collectors over a 12 month period..We are asking suppliers/ manufactures todonate collectors for testing. At present, westill need to raise a further 3500.00 fortest equipment, hot water cylinders, andother installation accessories. If you wouldlike to support our work, please take out asubscription to the magazine and/or makean individual or corporate donation to The

Sustainability Institute. Any help, no mat-ter how small, will be greatly appreciated.

The Institute also runs workshops onDIY Solar Thermal. Further details areelsewhere in this magazine and on ourwebsite. A DIY solar collector made fromrecycled domestic radiators erected on asite near Westport has circulated water atover 90C!!

Fig 7 Using two cylinders in series. VolkerQuaschning

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