solar heaters

8/13/2019 Solar Heaters

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Abstract

A parabolic trough solar collector uses a mirror in the shapeof a parabolic cylinder to reflect and concentrate sunradiations towards a receiver tube located at the focus lineof the parabolic cylinder. The receiver absorbs theincoming radiations and transforms them into thermalenergy,the latter being transported and collected by a fluidmedium circulating within the receiver tube.This method ofconcentrated solar collection has the advantage of highefficiency and low cost,and can be used either for thermalenergy collection, for generating electricity or forboth,therefore it is an important way to exploit solar energydirectly.


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Solar heaters and parabolic devices

Solar water heating(SWH systems comprise

several innovations and many mature renewableenergy(or SHW Solar Hot Water technologieswhich have been accepted in most countries for manyyears. SWH has been widely used in !reece, Tur"ey,#srael, Australia, $apan, Austria and %hina.

#n a &close'coupled& SWH system the storage tan" is

horiontally mounted immediately above the solarcollectorson the roof. )o pumping is re*uired as thehot water naturally rises into the tan" throughthermosiphonflow. #n a &pump'circulated& systemthe storage tan" is ground or floor mounted and is
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below the level of the collectors+ a circulating pumpmoves water or heat transfer fluid between the tan"and the collectors.

SWH systems are designed to deliver the optimumamount of hot water for most of the year. However,in winter there sometimes may not be sufficient solarheat gain to deliver sufficient hot water. #n this case agas or electric booster is normally used to heat thewater

Overview

Hot water heated by the sun is used in many ways.

While perhaps best "nown in a residential setting toprovide hot domestic water, solar hot water also hasindustrial applications, e.g. to generate electricity.-

/esigns suitable for hot climates can be muchsimpler and cheaper, and can be considered anappropriate technologyfor these places. The globalsolar thermal mar"et is dominated by %hina, 0urope,

$apan and #ndia. #n order to heat water using solarenergy, a collector, often fastened to a roof or a wallfacing the sun, heats wor"ing fluidthat is either
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pumped (active system or driven by naturalconvection(passive system through it. The collectorcould be made of a simple glass topped insulated box

with a flat solar absorber made of sheet metalattached to copper pipes and painted blac", or a set ofmetal tubes surrounded by an evacuated (nearvacuum glass cylinder. #n industrial cases a

parabolic mirror can concentrate sunlight on the tube.Heat is stored in a hot water storage tan".

The volume of this tan" needs to be larger with solarheating systems in order to allow for bad weather,and because the optimum final temperature for the

solar collector is lower than a typical immersion orcombustion heater. The heat transfer fluid (HT1 forthe absorber may be the hot water from the tan", butmore commonly (at least in active systems is aseparate loop of fluid containing anti'freeeand acorrosion inhibitorwhich delivers heat to the tan"through a heat exchanger(commonly a coil of copper

tubing within the tan". Another lower'maintenanceconcept is the 2drain'bac"23 no anti'freee is re*uired+instead all the piping is sloped to cause water to drain

bac" to the tan". The tan" is not pressuried and isopen to atmospheric pressure. As soon as the pump
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shuts off, flow reverses and the pipes are emptybefore freeing could occur.

4esidential solar thermal installations fall into twogroups3 passive (sometimes called &compact& andactive (sometimes called &pumped& systems. 5othtypically include an auxiliary energy source (electricheating element or connection to a gas or fuel oilcentral heating system that is activated when thewater in the tan" falls below a minimum temperature

setting such as 667%. Hence, hot water is alwaysavailable. The combination of solar water heating andusing the bac"'up heat from a wood stove chimney toheat water8can enable a hot water system to wor"all year round in cooler climates, without thesupplemental heat re*uirement of a solar waterheating system being met with fossil fuels orelectricity.

When a solar water heating and hot'water centralheating system are used in con9unction, solar heatwill either be concentrated in a pre'heating tan" thatfeeds into the tan" heated by the central heating, orthe solar heat exchanger will replace the lowerheating element and the upper element will remain in

place to provide for any heating that solar cannotprovide. However, the primary need for centralheating is at night and in winter when solar gain is
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lower. Therefore, solar water heating for washing andbathing is often a better application than centralheating because supply and demand are better

matched. #n many climates, a solar hot water systemcan provide up to :6; of domestic hot water energy.This can include domestic non'electric concentratingsolar thermalsystems. #n many northern 0uropeancountries, combined hot water and space heatingsystems (solar combisystems are used to provide -6to 86; of home heating energy

History

There are records of solar collectors in the s therewas a surge of interest in solar heating in )orthAmerica after -=@>, but specially after the -=B? oilcrisis.
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The world saw a rapid growth of the use of solarwarm water after -=@>, with systems being mar"etedalso in $apan and Australia?Technical innovation

has improved performance, life expectancy and easeof use of these systems. #nstallation of solar waterheating has become the norm in countries with anabundance of solar radiation, li"e the Cediterranean,:and $apan and Austria, where there %olombiadeveloped a local solar water heating industry than"sto the designs of Das !aviotas, directed by EaoloDugari. /riven by a desire to reduce costs in socialhousing, the team of !aviotas studied the bestsystems from #srael, and made adaptations as to meetthe specifications set by the 5anco %entralHipotecario (5%H which prescribed that the systemmust be operational in cities li"e 5ogotF where there

are more than 8>> days overcast. The ultimatedesigns were so successful that Das !aviotas offeredin -=:G a 86 year warranty on any of its installations.ver G>,>>> were installed, and still function a*uarter of a century later.

#n 8>>6, Spain became the first country in the world

to re*uire the installation ofphotovoltaicelectricitygeneration in new buildings, and the second (after#srael to re*uire the installation of solar waterheating systems in 8>>@.=
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Australia has a variety of incentives (national andstate and regulations (state for solar thermalintroduced starting with C40T in -==B.->---8

Solar water heating systems have become popular in%hina, where basic models start at around -,6>> yuan(, much cheaper than in Western countries(around :>; cheaper for a given sie of collector. #tis said that at least ?> million %hinese householdsnow have one, and that the popularity is due to the

efficient evacuated tubes which allow the heaters tofunction even under gray s"ies and at temperatureswell below freeing.-?#srael and %yprus are thepercapitaleaders in the use of solar water heatingsystems with over ?>;'G>; of homes using them.-G
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Types of Solar Water Heating

The type and complexity of a solar water heating

system is mostly determined by3 The changes in ambient temperature during the

day'night cycle. %hanges in ambient temperature and solar

radiation between summer and winter. The temperature of the water re*uired from the

system.The minimum efficiency of the system is determined

by the amount or temperature of hot water re*uiredduring winter (when the largest amount of hot wateris often re*uired. The maximum efficiency of thesystem is determined by the need to prevent the water

in the system from becoming too hot (to boil, in anextreme case. There are two main categories of solarwater heating systems. Eassive systems rely onconvection or heat pipes to circulate water or heatingfluid in the system, while active systems use a pump.#n addition, there are a number of other systemcharacteristics that distinguish different designs3

The type of collector used (see below The location of the collector ' roof mount,

ground mount, wall mount


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The location of the storage tan" in relation to thecollector

The method of heat transfer' open'loop or

closed'loop (via heat exchanger Ehotovoltaic thermal hybrid solar collectors can

be designed to produce both hot water andelectricity.

Passive systems

A special type of passive system is the IntegratedCollector Storage(#%S or 5atch Heater where thetan" acts as both storage and solar collector. 5atchheaters are basically thin rectilinear tan"s with glassin front of it generally in or on house wall or roof.They are seldom pressurised and usually depend ongravity flow to deliver their water. They are simple,

efficient and less costly than plate and tube collectorsbut are only suitable in moderate climates with goodsunshine.

A step up from the #%S is the Convection HeatStorage unit(%HS or thermosiphon. These areoften plate type or evacuated tube collectors with

built'in insulated tan"s. The unit uses convection(movement of hot water upward to move the waterfrom collector to tan". )either pumps nor electricityare used to enforce circulation. #t is more efficientthan an #%S as the collector heats a small(er amount
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of water that constantly rises bac" to the tan". #t canbe used in areas with less sunshine than the #%S. An%HS also "nown as a compact system or monobloc

has a tan" for the heated water and a solar collectormounted on the same chassis. Typically thesesystems will function by natural convection or heat

pipes to transfer the heat energy from the collector tothe tan".

A special type of passive system is the Integrated

Collector Storage(#%S or 5atch Heater where thetan" acts as both storage and solar collector. 5atchheaters are basically thin rectilinear tan"s with glassin front of it generally in or on house wall or roof.They are seldom pressurised and usually depend ongravity flow to deliver their water. They are simple,efficient and less costly than plate and tube collectors

but are only suitable in moderate climates with goodsunshine.

A step up from the #%S is the Convection HeatStorage unit(%HS or thermosiphon. These areoften plate type or evacuated tube collectors with

built'in insulated tan"s. The unit uses convection(movement of hot water upward to move the waterfrom collector to tan". )either pumps nor electricityare used to enforce circulation. #t is more efficientthan an #%S as the collector heats a small(er amount
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of water that constantly rises bac" to the tan". #t canbe used in areas with less sunshine than the #%S. An%HS also "nown as a compact system or monobloc

has a tan" for the heated water and a solar collectormounted on the same chassis. Typically thesesystems will function by natural convection or heat

pipes to transfer the heat energy from the collector tothe tan".

Active systems

Active solar hot water systems employ a pump tocirculate water or HT1 between the collector and thestorage tan". Di"e their passive counterparts, activesolar water heating systems come as two types3 directactive systems pump water directly to the collector

and bac" to the storage tan" (direct collectors cancontain conventional freee'vulnerable metal pipes orlow pressure freee'tolerant silicone rubber pipes,indirect active systems which are usually made ofmetals pump heat transfer fluid (HT1, the heat ofwhich is transferred to the water in the storage tan".5ecause the pump should only operate when the fluidin the collector is hotter than the water in the storagetan", a controller is re*uired to turn the pump on andoff. The use of an electronically controlled pump hasseveral advantages3


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The storage tan" can be situated lower than thecollectors. #n passive systems the storage tan"must be located above the collector so that the

thermosiphon effect can transport water or HT1from collector to tan". The use of a pump allowsthe storage tan" to be located lower than thecollector since the circulation of water or HT1 isenforced by the pump. A pumped system allowsthe storage tan" to be located out of sight.

5ecause of the fact that active systems allowfreedom in the location of the storage tan", thetan" can be located where heat loss from the tan"is reduced, e.g. inside the roof of a house. Thisincreases the efficiency of the solar water heatingsystem.

)ew active solar water heating systems canma"e use of an existing warm water storagetan"s (&geysers&, thus avoiding duplication ofe*uipment.

4educing the ris" of overheating. #f no waterfrom the solar hot water system is used (e.g.

when water users are away, the water in thestorage tan" is li"ely to overheat. Several pumpcontrollers avoid overheating by activating the

pump during the day at during times of lowsunlight, or at night. This pumps hot water or


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HT1 from the storage tan" through the collector(which can be cool in low light levels, thuscooling the water in the storage tan".

4educing the ris" of freeing. 1or direct activesystems in cold weather, where freee tolerantcollectors or drain down approaches are notused, the pump controller can pump hot waterfrom the water storage tan" through the collectorin order to prevent the water in the collector from

freeing, thus avoiding damage to the metal partsof the system.

Active systems can tolerate higher watertemperatures than would be the case in an e*uivalent

passive system. %onse*uently active systems areoften more efficient than passive systems but are

more complex, more expensive, more difficult toinstall and rely on either mains or EJ sourcedelectricity to run the pump and controller.

System cost

#n sunny, warm locations, where freee protection isnot necessary, an #%S (batch type solar water heatercan be extremely cost effective.GG#n higher latitudes,there are often additional design re*uirements forcold weather, which add to system complexity. This
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has the effect of increasing the initialcost (but notthe life'cyclecost of a solar water heating system, toa level much higher than a comparable hot water

heater of the conventional type. The biggest singleconsideration is therefore the large initial financialoutlay of solar water heating systems.G6ffsettingthis expense can ta"e several yearsG@and the

paybac" period is longer in temperate environmentswhere the insolation is less intense.GBWhencalculating the total cost to own and operate, a properanalysis will consider that solar energy is free, thusgreatly reducing the operating costs, whereas otherenergy sources, such as gas and electricity, can be*uite expensive over time. Thus, when the initialcosts of a solar system are properly financed andcompared with energy costs, then in many cases the

total monthly cost of solar heat can be less than othermore conventional types of hot water heaters (also incon9unction with an existing hot water heater. Athigher latitudes, solar heaters may be less effectivedue to lower solar energy, possibly re*uiring largerandKor dual'heating systems.G:#n addition, federaland local incentives can be significant.

The calculation of long term cost and paybac" periodfor a household SWH system depends on a number offactors. Some of these are3
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Erice of purchasing solar water heater (morecomplex systems are more expensive

0fficiency of SWH system purchased

#nstallation cost State or government subsidy for installation of a

solar water heater Erice of electricity per "W.h )umber of "W.h of electricity used per month by

a household Annual tax rebates or subsidy for using

renewable energy Annual maintenance cost of SWH system Savings in annual maintenenance of

conventional (electricKgasKoil water heatingsystem

The following table gives some idea of the cost andpaybac" period to recover the costs. #t does not ta"einto account annual maintenance costs, annual taxrebates and installation costs. However the table doesgive an indication of the total cost and the order ofmagnitude of the paybac" period. The table assumesan energy savings of -G> "W.h per month (about

G.@ "W.hKday due to SWH.


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households and regions. 1or instance in central andsouthern 1loridathe paybac" period could easily be Byears or less rather than the 8- years indicated on the

chart for the


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Photo ! Two photos of a small solar heater made

from plastic mirror sheet bent into a parabola

shape by a wooden template! There is a blac"

temperature data logger at the focus which is

where the solar energy is being concentrated!

Apart from the screws and plastic mirror all the


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parabola #bits# were made from scrap! The shape

of the parabola is given by e$uation %&' see table

!

The Parabola

When Sun light (parallel rays falls onto a correctlyaligned parabolic mirror it will be reflected bac"towards a single point "nown as the focus. The

parabolic mirror therefore directs the energy arrivingover its surface to a 2hot spot2 at the focus. Lou can

use this energy to heat something.The basic shape of a parabola is derived from thee*uation3

yM N Gax -

Where y represents the distance away from the mirror

centre and x represents the 2height above2 the centre,see figure - and 8. The constant a is "nown as thefocal length ' the distance from the origin to the focus

point.


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(igure ! The basic geometry of a parabolamirror! )ight rays coming from infinity will tend

to be reflected towards a point called the focus!

#f we want to ma"e a parabolic reflector to our ownparticular specification i.e. in terms of its sie / (i.e.having maximum y N /K8, height h and focal lengtha, then e*uation - becomes3


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a N /MK-@h 8

A simple curved plane (rather than dish shape solar

heater is shown in photo - and we shall use this as anexample, of course what follows can also be used fora full 2dish shaped2 parabola and for much largerdesigns.

!

To ma"e it easy to locate the focal point when we areusing it, # chose to design the mirror surface height

(h to be the same value as a ' the focus distanceabove the canter of the mirror (so a N h in thisexample -- cm. #n this case finding the focus issimple ' imagine a line going across the top of thedevice, the centre of this line is where the focus is.


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4earranging e*uation 8 we get the diameter K lengthof the solar heater / to be3

/M N -@ah, so using h N a N -- cm we get3/ N O(-@x--x-- N O(-=?@ N GG cm

We have yM N Gax and a N -- cm1or the solar heater shown in photo - we get x NyMKGG? (all dimensions in cm

)ow that we have chosen the basic *uantities for theparabola we can compute the x and y coordinatesfrom e*uation - and ?. Some of these are tabulated intable -. To re'cap, the y axis represents the distanceaway from the mirrors centre while x represents the2height2 above the base, the lowest point of the mirror

(its centre.Lou can also ma"e a parabola support by cutting twoside (parabola shaped templates, see photo -. #n thiscase when mar"ing up simply add a few cm (9 infigure 8 to all the x values otherwise the templatewill diminish to ero thic"ness at the centre and itwill all fall apartP


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Table * y and + values derived from e$uation ,

for the parabolic solar heater shown in photo !

-y .cm/ y0 y0122 + .cm/> > > >

- - -KGG >

8 G GKGG >.-

? = =KGG >.8

G -@ -@KGG >.?6 86 86KGG >.@

@ ?@ ?@KGG >.:

B G8 G8KGG >.=

: @G @GKGG -.6

... ... ... ...8> G>> G>>KGG =.-

and so on for larger values of y


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3ough Science solar heater

#n the second 55% TJ 4ough Science series 0llenCc%allie and # made a large solar heater. A piece ofstraight wooden plan" was cut for the base (/N8m.The central half way point was mar"ed. At Qy cmdistances every -6 cm or so, from the centre, vertical

battens were attached having heights of x cm. Whenthese were all in place a thin piece of plywood was

put over the top to complete the parabolic surface.ur mirror had a surface area of about -m8.

4irrors

Jarious mirror surfaces can be used to cover theparabola surface. The 4ough Science device usedfragments of bro"en mirror cemented to the parabolic

surface (see )ote - below. # have also used small ?cm mirror s*uares from an arts supplier as well asaluminium "itchen foil (see note G. 1lexiblemirrored plastic sheet is also available which has


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excellent properties, it can also be cut with scissorsand is very safe, easy to manipulate and change.

)ocating the Sun and setting up the devicebviously there needs to be direct sunlight to get anydecent heating from the reflector but it is alsoimportant that the device is correctly aligned to theSun. #f the base can be extended a little and a boltfixed through so that it points out at right angles thenthe bolts shadow should disappear when the device is

pointing directly at the Sun. bviously you can alsoad9ust the hot spot for maximum brightness by trialand error.

What to heat5

At the e*uator there is about -"W of solar power forevery s*uare meter of collecting surface. The small

heater shown in photo - only has an area of about-K8>th mM but it still ma"es a good demonstration.Darger parabolic system having say -'8 mM surfacearea should have enough energy to boil water in asmall "ettle for a cup of teaP The amount of heatingobviously depends on the time of year K day, theweather and 2strength2 of the sun (your latitude.Eainting the "ettle mat blac" with fire proof painthelps. Eutting a large transparent plastic bag aroundthe "ettle also helps reduce heat loss (it acts li"e a


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mini green house. #t would be fun to try coo"ing anegg on a flat plate arranged at the focus.

Satellite dish * a section of a much larger

parabolic surface

The satellite dish

0nergy falling on to a correctly aligned parabolicmirror (of collecting surface area A will be reflectedto the focal point. However if you heat somethinglarge at this focus it will bloc" the light getting to the


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mirror, effectively reducing A. There is a way ofgetting around this problem however.

#magine a giant parabolic dish most of which we cutaway to leave behind 9ust a parabolic section of areaA, near to the circumference. This remaining sectionwill collect the same amount of energy as an ideal(full parabola of the same area. Any energy fallingon to this section would still be reflected to the focus

point although now this point seems rather off'set

from the dish. This is a useful modification ase*uipment put at the focal point will now not bloc"the arriving energy. This techni*ue is often used insatellite TJ receiving dishes where the TJ receiverhead (a small electronic device is placed at the focus(which now seems off'set so the e*uipment does not

bloc" the path of the wea" TJ signals coming infrom the space satellites (see above.

Solar Water Heater - Applications

/ 6omestic 7 1lats, 5ungalows and Apartments.

8/ Commercial 7Hotels, Hospitals, Hostels and/ormitories.


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,/ Industrial 7Erocess #ndustries, Ereheating boilerfeed water. #n domestic sector, hot water is used for

bathing, washing of clothes R utensils etc. The

re*uirement may, however, vary with the season ofthe year R numberof family members. urexperience says that on an average ?> to ?6 litres ofwater at 6> to 66 %. is consumed by an individual.Thus for a family of G members, -86 DE/ SolarWater Heating System is *uite sufficient.

#n commercial R industrial sectors, where large*uantity of water is re*uired at fairly hightemperature, 22$ain Solar Water Heating Systems22 aredesigned to meet the above re*uirement. /ependingon the distribution pattern of hot water, the systemcould be either modular or a big capacity single tan"system.

solar heaters

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