solar heaters (2)

8/7/2019 Solar heaters (2)

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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 Greece, Turkey,Israel, Australia, Japan, Austria and China.

In a "close-coupled" SWH system the storage tank is

horizontally mounted immediately above the solarcollectors on the roof. No pumping is required as thehot water naturally rises into the tank throughthermosiphon flow. In a "pump-circulated" systemthe storage tank 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 tankand 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. In 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 known in a residential setting toprovide hot domestic water, solar hot water also hasindustrial applications, e.g. to generate electricity.[1]

Designs suitable for hot climates can be muchsimpler and cheaper, and can be considered anappropriate technology for these places. The globalsolar thermal market is dominated by China, Europe,

Japan and India.In order to heat water using solar

energy, a collector, often fastened to a roof or a wallfacing the sun, heats working fluid that 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 black, or a set ofmetal tubes surrounded by an evacuated (nearvacuum) glass cylinder. In industrial cases a

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

The volume of this tank 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 (HTF) forthe absorber may be the hot water from the tank, butmore commonly (at least in active systems) is aseparate loop of fluid containing anti-freeze and acorrosion inhibitorwhich delivers heat to the tankthrough a heat exchanger(commonly a coil of copper

tubing within the tank). Another lower-maintenanceconcept is the 'drain-back': no anti-freeze is required;instead all the piping is sloped to cause water to drain

back to the tank. The tank is not pressurized and isopen to atmospheric pressure. As soon as the pump
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shuts off, flow reverses and the pipes are emptybefore freezing could occur.

Residential solar thermal installations fall into twogroups: passive (sometimes called "compact") andactive (sometimes called "pumped") systems. Bothtypically include an auxiliary energy source (electricheating element or connection to a gas or fuel oilcentral heating system) that is activated when thewater in the tank falls below a minimum temperature

setting such as 55C. Hence, hot water is alwaysavailable. The combination of solar water heating andusing the back-up heat from a wood stove chimney toheat water[2] can enable a hot water system to workall year round in cooler climates, without thesupplemental heat requirement of a solar waterheating system being met with fossil fuels orelectricity.

When a solar water heating and hot-water centralheating system are used in conjunction, solar heatwill either be concentrated in a pre-heating tank thatfeeds into the tank 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. In many climates, a solar hot water systemcan provide up to 85% of domestic hot water energy.This can include domestic non-electric concentratingsolar thermal systems. In many northern Europeancountries, combined hot water and space heatingsystems (solar combisystems) are used to provide 15to 25% of home heating energy

History

There are records of solar collectors in the United

States dating back to before 1900,[3] comprising ablack-painted tank mounted on a roof. In 1896Clarence Kemp of Baltimore, USA enclosed a tank ina wooden box, thus creating the first 'batch waterheater' as they are known today. Although flat-platecollectors for solar water heating were used inFlorida and Southern California in the 1920s therewas a surge of interest in solar heating in NorthAmerica after 1960, but specially after the 1973 oilcrisis.
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The world saw a rapid growth of the use of solarwarm water after 1960, with systems being marketedalso in Japan and Australia[3] Technical innovation

has improved performance, life expectancy and easeof use of these systems. Installation of solar waterheating has become the norm in countries with anabundance of solar radiation, like the Mediterranean,[8] and Japan and Austria, where there Colombiadeveloped a local solar water heating industry thanksto the designs ofLas Gaviotas, directed by PaoloLugari. Driven by a desire to reduce costs in socialhousing, the team of Gaviotas studied the bestsystems from Israel, and made adaptations as to meetthe specifications set by the Banco CentralHipotecario (BCH) which prescribed that the systemmust be operational in cities like Bogot where there

are more than 200 days overcast. The ultimatedesigns were so successful that Las Gaviotas offeredin 1984 a 25 year warranty on any of its installations.Over 40,000 were installed, and still function aquarter of a century later.

In 2005, Spain became the first country in the world

to require the installation ofphotovoltaic electricitygeneration in new buildings, and the second (afterIsrael) to require the installation of solar waterheating systems in 2006.[9]
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Australia has a variety of incentives (national andstate) and regulations (state) for solar thermalintroduced starting with MRET in 1997.[10][11][12]

Solar water heating systems have become popular inChina, where basic models start at around 1,500 yuan(US$190), much cheaper than in Western countries(around 80% cheaper for a given size of collector). Itis said that at least 30 million Chinese householdsnow have one, and that the popularity is due to the

efficient evacuated tubes which allow the heaters tofunction even under gray skies and at temperatureswell below freezing.[13] Israel and Cyprus are thepercapita leaders in the use of solar water heatingsystems with over 30%-40% of homes using them.[14
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Types of Solar Water Heating

The type and complexity of a solar water heating

system is mostly determined by: The changes in ambient temperature during the

day-night cycle. Changes in ambient temperature and solar

radiation between summer and winter. The temperature of the water required from the

system.The minimum efficiency of the system is determined

by the amount or temperature of hot water requiredduring winter (when the largest amount of hot wateris often required). 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. Passive systems rely onconvection or heat pipes to circulate water or heatingfluid in the system, while active systems use a pump.In addition, there are a number of other systemcharacteristics that distinguish different designs:

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 tank in relation to thecollector

The method ofheat transfer- open-loop or

closed-loop (via heat exchanger) Photovoltaic 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 (ICS or Batch Heater) where thetank acts as both storage and solar collector. Batchheaters are basically thin rectilinear tanks 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 ICS is the Convection HeatStorage unit (CHS orthermosiphon). These areoften plate type or evacuated tube collectors with

built-in insulated tanks. The unit uses convection(movement of hot water upward) to move the waterfrom collector to tank. Neither pumps nor electricityare used to enforce circulation. It is more efficientthan an ICS as the collector heats a small(er) amount
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of water that constantly rises back to the tank. It canbe used in areas with less sunshine than the ICS. AnCHS also known as a compact system ormonobloc

has a tank 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 tank.

A special type of passive system is the Integrated

Collector Storage (ICS or Batch Heater) where thetank acts as both storage and solar collector. Batchheaters are basically thin rectilinear tanks 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 ICS is the Convection HeatStorage unit (CHS orthermosiphon). These areoften plate type or evacuated tube collectors with

built-in insulated tanks. The unit uses convection(movement of hot water upward) to move the waterfrom collector to tank. Neither pumps nor electricityare used to enforce circulation. It is more efficientthan an ICS as the collector heats a small(er) amount
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of water that constantly rises back to the tank. It canbe used in areas with less sunshine than the ICS. AnCHS also known as a compact system ormonobloc

has a tank 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 tank.

Active systems

Active solar hot water systems employ a pump tocirculate water or HTF between the collector and thestorage tank. Like their passive counterparts, activesolar water heating systems come as two types: directactive systems pump water directly to the collector

and back to the storage tank (direct collectors cancontain conventional freeze-vulnerable metal pipes orlow pressure freeze-tolerant silicone rubber pipes),indirect active systems which are usually made ofmetals pump heat transfer fluid (HTF), the heat ofwhich is transferred to the water in the storage tank.Because the pump should only operate when the fluidin the collector is hotter than the water in the storagetank, a controller is required to turn the pump on andoff. The use of an electronically controlled pump hasseveral advantages:


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The storage tank can be situated lower than thecollectors. In passive systems the storage tankmust be located above the collector so that the

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

Because of the fact that active systems allowfreedom in the location of the storage tank, thetank can be located where heat loss from the tankis reduced, e.g. inside the roof of a house. Thisincreases the efficiency of the solar water heatingsystem.

New active solar water heating systems canmake use of an existing warm water storagetanks ("geysers"), thus avoiding duplication ofequipment.

Reducing the risk of overheating. If no waterfrom the solar hot water system is used (e.g.

when water users are away), the water in thestorage tank is likely 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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HTF from the storage tank through the collector(which can be cool in low light levels), thuscooling the water in the storage tank.

Reducing the risk of freezing. For direct activesystems in cold weather, where freeze tolerantcollectors or drain down approaches are notused, the pump controller can pump hot waterfrom the water storage tank through the collectorin order to prevent the water in the collector from

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

Active systems can tolerate higher watertemperatures than would be the case in an equivalent

passive system. Consequently active systems areoften more efficient than passive systems but are

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

System cost

In sunny, warm locations, where freeze protection isnot necessary, an ICS (batch type) solar water heatercan be extremely cost effective.[44] In higher latitudes,there are often additional design requirements forcold weather, which add to system complexity. This
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has the effect of increasing the initialcost (but notthe life-cycle cost) 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.[45] Offsettingthis expense can take several years[46] and the

payback period is longer in temperate environmentswhere the insolation is less intense.[47] Whencalculating 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 bequite 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 inconjunction with an existing hot water heater). Athigher latitudes, solar heaters may be less effectivedue to lower solar energy, possibly requiring largerand/or dual-heating systems.[48] In addition, federaland local incentives can be significant.

The calculation of long term cost and payback periodfor a household SWH system depends on a number offactors. Some of these are:
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Price of purchasing solar water heater (morecomplex systems are more expensive)

Efficiency of SWH system purchased

Installation cost State or government subsidy for installation of a

solar water heater Price of electricity per kW.h Number of kW.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 (electric/gas/oil) water heatingsystem

The following table gives some idea of the cost andpayback period to recover the costs. It does not takeinto 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 payback period. The table assumesan energy savings of 140 kW.h per month (about

4.6 kW.h/day) due to SWH. Unfortunately paybacktimes can vary greatly due to regional sun, extra costdue to frost protection needs of collectors, householdhot water use etc. so more information may beneeded to get accurate estimates for individual


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households and regions. For instance in central andsouthern Florida the payback period could easily be 7years or less rather than the 21 years indicated on the

chart for the US.[49]

Parabolic Water Heater arrangment

Parabolic mirror surfaces are found wherever energyneeds to be focused efficiently and accurately. Youfind them built into car headlights, spotlights as well

as astronomical telescopes. Here we explore theparabola to design solar heaters.
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Photo 1. Two photos of a small solar heater made

from plastic mirror sheet bent into a parabola

shape by a wooden template. There is a black

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 equation [1], see table

1.

The Parabola

When Sun light (parallel rays) falls onto a correctlyaligned parabolic mirror it will be reflected backtowards a single point known as the focus. The

parabolic mirror therefore directs the energy arrivingover its surface to a 'hot spot' at the focus. You can

use this energy to heat something.The basic shape of a parabola is derived from theequation:

y = 4ax [1]

Where y represents the distance away from the mirror

centre and x represents the 'height above' the centre,see figure 1 and 2. The constant a is known as thefocal length - the distance from the origin to the focus

point.


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Figure 1. The basic geometry of a parabolamirror. Light rays coming from infinity will tend

to be reflected towards a point called the focus.

If we want to make a parabolic reflector to our ownparticular specification i.e. in terms of its size D (i.e.having maximum y = D/2), height h and focal lengtha, then equation 1 becomes:


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a = D/16h [2]

A simple curved plane (rather than dish shape) solar

heater is shown in photo 1 and we shall use this as anexample, of course what follows can also be used fora full 'dish shaped' parabola and for much largerdesigns.

.

To make it easy to locate the focal point when we areusing it, I 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 = h in thisexample 11 cm). In 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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Rearranging equation 2 we get the diameter / lengthof the solar heater D to be:

D = 16ah, so using h = a = 11 cm we get:D = (16x11x11) = (1936) = 44 cm

We have y = 4ax and a = 11 cmFor the solar heater shown in photo 1 we get x =y/44[3] (all dimensions in cm)

Now that we have chosen the basic quantities for theparabola we can compute the x and y coordinatesfrom equation 1 and 3. Some of these are tabulated intable 1. To re-cap, the y axis represents the distanceaway from the mirrors centre while x represents the'height' above the base, the lowest point of the mirror

(its centre).You can also make a parabola support by cutting twoside (parabola shaped) templates, see photo 1. In thiscase when marking up simply add a few cm (j infigure 2) to all the x values otherwise the templatewill diminish to zero thickness at the centre and itwill all fall apart!


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Table 1 - y and x values derived from equation 3

for the parabolic solar heater shown in photo 1.

y (cm) y y/44 x (cm)0 0 0 0

1 1 1/44 0

2 4 4/44 0.1

3 9 9/44 0.2

4 16 16/44 0.35 25 25/44 0.6

6 36 36/44 0.8

7 42 42/44 0.9

8 64 64/44 1.5

... ... ... ...20 400 400/44 9.1

and so on for larger values of y


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Rough Science solar heater

In the second BBC TV Rough Science series EllenMcCallie and I made a large solar heater. A piece ofstraight wooden plank was cut for the base (D=2m).The central half way point was marked. At y cmdistances every 15 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.Our mirror had a surface area of about 1m2.

Mirrors

Various mirror surfaces can be used to cover theparabola surface. The Rough Science device usedfragments of broken mirror cemented to the parabolic

surface (see Note 1 below). I have also used small 3cm mirror squares from an arts supplier as well asaluminium kitchen foil (see note 4). Flexiblemirrored 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.

Locating the Sun and setting up the deviceObviously 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. If 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. Obviously you can alsoadjust the hot spot for maximum brightness by trialand error.

What to heat?

At the equator there is about 1kW of solar power forevery square meter of collecting surface. The small

heater shown in photo 1 only has an area of about1/20th m but it still makes a good demonstration.Larger parabolic system having say 1-2 m surfacearea should have enough energy to boil water in asmall kettle for a cup of tea! The amount of heatingobviously depends on the time of year / day, theweather and 'strength' of the sun (your latitude).Painting the kettle mat black with fire proof painthelps. Putting a large transparent plastic bag aroundthe kettle also helps reduce heat loss (it acts like a


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mini green house). It would be fun to try cooking anegg on a flat plate arranged at the focus.

Satellite dish - a section of a much larger

parabolic surface

The satellite dish

Energy 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 block the light getting to the


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

Imagine a giant parabolic dish most of which we cutaway to leave behind just 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 asequipment put at the focal point will now not blockthe arriving energy. This technique is often used insatellite TV receiving dishes where the TV receiverhead (a small electronic device) is placed at the focus(which now seems off-set) so the equipment does not

block the path of the weak TV signals coming infrom the space satellites (see above).

Solar Water Heater - Applications

1) Domestic : Flats, Bungalows and Apartments.

2) Commercial : Hotels, Hospitals, Hostels andDormitories.


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3) Industrial : Process Industries, Preheating boilerfeed water. In domestic sector, hot water is used for

bathing, washing of clothes & utensils etc. The

requirement may, however, vary with the season ofthe year & numberof family members. Ourexperience says that on an average 30 to 35 litres ofwater at 50 to 55 C. is consumed by an individual.Thus for a family of 4 members, 125 LPD SolarWater Heating System is quite sufficient.

In commercial & industrial sectors, where largequantity of water is required at fairly hightemperature, ''Jain Solar Water Heating Systems'' aredesigned to meet the above requirement. Dependingon the distribution pattern of hot water, the systemcould be either modular or a big capacity single tanksystem.

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