Solar Water Heating and the Plant Engineer

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42

Solar waterheating and the

plant engineer

Tony BookRiomay Ltd

42

Contents

Introduction 42/3

42.1 Solar radiation and energy flows on earth 42/3

42.2 Spectral distribution of solar radiation 42/3

42.3 Distribution of solar power around the world 42/5

42.4 Seasonal variation in the UK 42/5

42.5 Background to basic connector circuits 42/6

42.6 The benefits of solar heating 42/7

42.7 How a solar collector works 42/8

42.8 Swimming pool or large volume water heating 42/9



Introduction

Whenever the term Solar Water heating gets mentioned,one's mind immediately focuses on the domestic scene,as this is the area where, as an individual, the plant engi-neer gets attacked with junk post and evening telephonesalespeople offering the earth and costing a price basedon some well conceived payback calculation formulae.

In a commercial organization or plant, the provision

of domestic washing or canteen water is one of thoseoverlooked services which tends to get neglected, for-gotten and probably, as a stand alone item, may remaina very expensive and neglected commodity, with elec-

tric water heaters, probably with incorrectly set ther-mostats, running all hours and costing a smallfortune.

Some plant engineers may be able to take ad vantage ofsurplus heat from steam or condensate systems, processexcesses, compressor cooling systems or some other formof heat recovery systems etc. in order to augm ent their hotwater provision costs.

Over the past few years there have been great stridesforward in the development of solar collecting deviceswhich range in design, cost effectiveness and efficiency

levels and overall developments.Apart from the provision of the domestic requirements

for hot or warm water there are many other requirementswithin comm ercial premises. Processes, wh ich cover quitea wide range of activities, or lend themselves to at least,be considered for solar heating applications. Laundries,dye w orks process wash ing and cleansing plants, althoughinitially provision by m eans of solar panels m ay not coverthe whole application, there may be occasions where

consideration may be given for joint systems, possiblyincorporating water to water, air to water heat recovery,and the use of a heat pumps in order to reach the desiredgoal and or even part of the way.

Systems vary in complexity from a solar panel consist-ing of a simple set of pipes within a black glass-frontedbox with a small pump coupled up for circulation pur-poses, to the latest versions where the panels can be ofthe flat type to cylindrical gas-filled units with specialcoatings used on the glass and the tubes to improve theirheat collection and heat transfer.

The methods of installation and configurations have

been developed further, the simplest are very much basedon the original system, although the control systems havedeveloped ev en further to take full advantage of what heattransfer is available.

The provision of heating and the use of solar applica-tions cannot be considered in isolation as the w hole aspectof heat and energy recovery, conservation and environ-mental acceptability.

In order to be able to give a full and sensible approachto Solar Heating it is imperative that the whole idea isapproached from the basics, and once understood thenappreciation will enable the various applications to be

given their full consideration.The w hole concept is thus founde d upon the effects anduse of Solar Radiation.

42.1 Solar radiation and energy flows onearth

Solar radiation is the heat and light and other radiationgiven off by the Sun. Nuclear reactions in the interior ofthe Sun maintain a central temperature of 16 million 0C,and a surface temperature of 570O

0C. Like all hot objects,

the Sun's surface radiates energy at a rate and a colour(wavelength range) wh ich depends on its temperature. The

Sun emits radiation at a rate of 3.8 x 1011 Watt, of whichonly two parts in a thousand million arrive at the Earth,with the rest disappearing into space or w arm ing the otherplanets in our solar system.

At the outer edge of the Earth's atmosphere, the solarradiation on each square metre amounts to 1350Watt,mainly of visible radiation and heat plus a little ultraviolet.Nearly one third of this is reflected back into space byclouds, water, ice etc. The rest of the solar radiation isabsorbed and provides the energy for almost all the naturalprocesses on Earth, as shown in Figure 42.1. The energyis shown in units of GigaWatt (GW ) - about equal to the

output of a large electricity generating station.The average temperature of the Earth's surface (aver-aged over day and night and the seasons over the wholesurface from equator to poles) has remained remarkablyconstant over the past few thousand million years. It hasvaried by only a few degrees from the mean of about14 0C or 287K. Any body at any temperature, includ-ing the human body, emits radiation. This is because italso absorbs energy from its environment (e.g. from thesun) and without emitting this energy back it would heatup infinitely. This emission is mainly long wavelengthinfrared (heat) radiation.

To maintain a con stant temperature, the Earth's surface

must emit the same amount of energy into space (in theform of heat radiation) as it receives from the Sun. Notethat in Figure 42.1, the total input of 173 million GWis balanced by the 5 2 million GW reflected into spaceand the 121 million GW emitted as heat radiation by theEarth's surface. The temperature of the Earth's surface isdetermined by this balance of radiation. At night whenthere is no input, the surface temperature falls as someenergy is radiated back into space. A cloudy sky reflectsmuch of the heat radiation back to the ground, so clearnights are colder than cloudy ones. Some atmosphericgases such as carbon dioxide or m ethane can trap the long

wavelength radiation. Since less energy is radiated awaythan is being absorbed from the Sun, then the temperatureof the Earth will rise - this is the Greenhouse Effect.

42.2 Spectral distribution of solarradiation

Solar radiation extends over a wide range of wavelengths(colours) of light. The precise spectrum depends on thepath length of the sunlight through the atmosphere, andthe humidity, dust content etc., of the air. Figure 42.2shows the spectrum of sunlight for the Sahara Desert

when the sun is directly overhead, and for a typical Britishsunny summer's day. The dips in the solar spectrum aredue to absorption of the sunlight on its way through the



Figure 42.2 Spectra l distributionof sunlight for the Sahara desert and a British summer's da y

Ultra-violet InfraredWavelength (jim)

British summer day

Sahara desert

Eyeresponse

Figure 42.1 Energy Flows of the Planet Earth (units = gigawa tts, GW )

Fossil

Fuels

Nuclear, Thermal and

Gravitational energy

Earth

Storage

in plants

Fuels for human needs Terrestrial energy

Earth

Conduction through rocks

Volcanic activity

Tides, tidal currents

Input

Tidal

Energy

3,000

Output

Long-wave

Radiation

121 million

Output

Short-wave

Radiation

52 million

Input

Solar

Radiation

173 million

Direct reflection

Direct heating 81 mllion

Evaporation of water, etc

40 mllionTemporary storage

in water and ice

Energy in wind and waves

PhotosynthesisDecay

Animals

Bu

Ge

Y

ow

Re

S

aE

g(W

a

)



atmosphere, by ozone in the ultraviolet and by watervapour and carbon dioxide in the infrared. For comparisonthe spectrum of the long-wavelength radiation emittedby the Earth would be a broad curve with a peak atabout 10 urn.

The response curve of the human eye is also shown inFigure 42.2and it is interesting to see how closely hum aneyes (and those of many other animals) have adapted tohave a maximum sensitivity where there is most energy

in the sunlight.

42.3 Distribution of solar power aroundthe world

Solar radiation is very variable. It varies from place toplace, and from time to time. As we move from thepoles towards the equator, the power of the sunlightincreases. This is partly because the sun is more directlyoverhead, and therefore has a shorter path length throughthe atmosphere, an d partly because there are more clouds

at higher latitudes. There is in fact less solar energy at theequator than at the tropics because of the equatorial rainclouds. Figure 42.3 shows the average solar pow er aroundthe world. These power data are obtained by measuringthe average solar energy received in one year and dividingby the number of seconds in a year, so the averagesinclude day and night. Note that in the UK we receivean average power of about 125Wm~

2 whilst the SaharaDesert gets 250-300 W m~

2- a much smaller difference

than most people would guess.North of the equator, the highest radiation levels in the

world are found in the Sahara desert, the Arabian Gulf

area, and the deserts of C alifornia and N ew M exico. Southof the equator the Kalahari and Australian desert areashave the highest levels. These areas al l average about250-300 W m ~

2. Southern Europe has a radiation level of

about 200W m~ 2, as does most of the "sun belt" of theworld, between latitudes 4O

0N an d 4O

0S.

42.4 Seasonal variation in the UK

The amount of sunshine varies with the seasons, an dthe higher the latitude the greater the seasonal variation.Figure 42.4shows the average energy received in the UK.

The variation between summer and winter is very large(a factor of over 10 in most places). Since we need moreenergy in winter because the temperature is lower andthe nights much longer, the energy received directly fromthe sun does not m atch the UK energy dem and at all well.Wind and wave energy are much more abundant in winterthan in summer, and these indirect solar energy sourcesare a much better match to UK energy demands.

The daily energies shown in Figure 42.4 are averagedover the whole of the particular month. The sunshine inany month varies from year to year, particularly in islandclimates such as ours, and the data in Figure 42.4 are fortypical months, i.e. averages over many years. For thesunniest months (which might happen 1 year in 10), thedaily average energy received can be twice the value inFigure 42.4,whilst for the worst months (again occurring1 year in 10), it can be as little as one fifth of thesevalues.

The data in this figure refer to the energy received ona horizontal surface, such as flat farmland. For passive

Figure 42.3 Average solar power around the World



solar architecture, we need to know the energy passingthrough vertical windows, whilst for solar heating orphotovoltaics, the surfaces are tilted to be nearly normalto the sun's rays. Figure 42.5 shows the areas projectedby a beam of sunlight on vertical and horizontal surfaces.Since these areas are bigger than the area of the beam,the power in the beam is spread over these larger surfacesand the power pe r unit area decreases. The power densityfor a bright sunny day in June and December at middayis shown in the table above. More pow er enters a windowin December, even though the sun light is weaker, but thepower to warm the soil or grow the crops is less than 113in December compared to June.Units: The usual unit of energy is the Joule, equalto a power of 1 Watt applied for 1 second, IJ =I W s . Another unit of energy that is often used is thekiloWatthour (kWh). Since there are 3600 seconds inan hour, 1 kWh = (1000 x 3600) Joule = 3.6MegaJoule(MJ). The average solar power at the Earth's surface, of

125 Wm"2

(Watt per square metre) translates to 3600 x24 x 125 = 10.8MJm-

2per day, equivalent to 10.8/3.6 =

3kWhm-2day-

1.

42.5 Background to basic collectorcircuits

In Britain each square metre of a south-facing roofreceives around 1000 kWh of solar radiation during a year.This means that the roofs of many of our homes receive

more energy from the sun in a year than we need to pro-vide both the space heating and hot water; by using solarcollectors it is possible to capture some of this solar radi-ation and reduce our consumption of fossil fuels like gas,coal an d oil.

The sun is used to provide hot water for a housein many countries. In Britain it is possible to use thesun to provide most of an average families hot waterrequirements from about May to September and to obtainsome 'pre-heating' of the cold water supply during theother months.

In principle it is possible to scale up the size of a solar

water heater to provide under floor heating and a solarwater heater can be used in a preheating arrangement ifthe produced ho t water is not used elsewhere.

Figure 42.5 Effect of slanting of sunlight on power density on vertical and horizontal surfaces

Horizontal (ground)

Vertical (window)

Surface tilted normalto the sun's rays Su n angleO

at noon

Surface

normal to the

sun's rays

Vertical

surface

(window)

Horizontal

surface(ground)

June

30°

900 Wm-2

450 Wm~2

780 Wm-2

December

70°

700 Wm-2

660 Wm~2

240 Wm~2

Figure 42.4 Daily average solar energy received in the UK (kWhm2da y

1)

March June September December

1/sinO

1/cos O

V

c



Figure 42.7 Illustration of a flat plate col lector

Hot water is normally produced by heating the coldmains water to the required temperature with a gas or oilfired boiler or an immersion heater. By slightly modifyingthe conventional heating system, solar collectors may beintroduced, as shown in Figure 42.6. The solar collectorsare securely attached to some convenient part of the houseso that they face south and are tilted from the horizontalaround 3O

0C. A pitched roof which faces between E.S.E.

and W.S.W. and is unshaded is usually the best locationfor the collectors.

A special water based anti-freeze solution is used inthe solar circuit to transfer the heat from the solar col-lectors to a heat exchanger in the hot water cylinder. A

controller switches on the circulating pump only whenthe collectors are hotter than the hot water cylinder,thus preventing the cylinder from being cooled at nightor on dull days. All the pipe work as well as thehot water cylinder are well insulated to reduce heatlosses.

42.6 The benefits of solar heating

Some people may install a solar water heating system to

help conserve the world's supply of fossil fuels. Mostpeople, ho wever, w ill be more concerned with the moneythat the system will save them in the future.

Fluid inlet

Blackenedabsorber plate

Heat transferfluid conduits

Fluid outlet

Durable casing

Transparent cover

Insulation

Figure 42.6 Typical plumbing arrangement

1. Twin coil 300 litre hot water cylinder2. Suntube solar panels

3. Electronic controller4. Circulating pump5. Non return valve6. Pressure tem p valve/air vent7. Immersion heater8. Mains in9. Electrical supply

10.Tank sensor11. Low sensor12. Gas boiler13. Basins14. High sensor



The cost of a solar water heater consists of:

the cost of the componentsthe cost of paying the installer to fit it

These costs represent your initial investment in thesystem. The savings made each year in conventionalheating fuels will repay your initial investment and thetime taken to get your money back is often called the

payback period. In the U K it is unlikely that the paybackperiod will be less than ten years hence it is important toinstall a well made system.

A solar water heating system for a typical family of twoadults and two children might have 3-4m

2of collector

and a hot water tank capacity of 150 litres. For familieswith smaller or larger ho t water needs these amounts canbe adjusted accordingly. In typical UK conditions sucha system will collect up to about 350kWh/year per m

2

of installed collector depending on the amount of waterused. Of course if you don't use much ho t water, youcan't save much energy, thus solar water heater are mosteconomically in places w ith large hot water dem ands, such

as schools and hospitals.

42.7 How a solar collector works

The majority of solar collectors used for heating domes-tic hot water are of the flat type with a transparent cover.These collectors often referred to as flat plate collectors

consist of a black absorber (which is rather like a cen-tral heating radiator) contained in a weatherproof box,insulated at the back and glazed at the front.

Since the collectors are located outside the house an dhave to operate fo r many years, it is important thatthe materials used in their construction are durable andcompatible with the rest of the plumbing. Since collectorscan reach temperatures in excess of 20O

0C if they are

left empty, all the materials in contact with the absorber

must be able to resist this temperature without m elting ordistorting.

Solar irradiationHeat emission of a body at 8O

0C

Figure 42.8 Comparison of solar and heat radiation

12 Automatic air ventIndirect solar heating system to swimmng pool using existing filtration plant

High sensor 11

Solar

pump

Expansion vessel

Heat exchanger Filtration RelaySolar

controller

Power supply

Time clock

Filtration

pump

Low sensor

Figure 42.9 Il lustration of a swimming pool system



A flat plate collector uses the well known 'greenhouseeffect'. The energy from the sun reaching the earth ismainly in the invisible w avelengths. The glazing material(glass or plastic) does not absorb these wavelengths to anysignificant extent and therefore most of the incoming radi-ation is received by the blackened absorber. The absorberincreases in temperature and the heat is conducted throughthe metal fins to the heat transfer fluid in the waterways.

Both the insulation below the absorber and the glazing

at the front reduce heat losses due to convection. Theheat radiation from the hot absorber plate is preventedfrom escaping directly back to the atmosphere by glazingwhich absorbs it .

When designing a collector, therefore, it is importantthat the glazing should have a high transmittance and thecoating on the absorber plate, a high absorption for solarradiation.

In addition, the thermal performance of the collectormay be improved by applying special coatings to theabsorber place which have the effect of reducing theradiation given off by the plate. Figure 42.8 sh ows the dif-

ference in the spectrum of a hot body and solar irradiation.The air conduction an d convection losses may bereduced by creating a vacuum between the glazing andabsorber plate like that of a thermos flask. However, theimprovements in solar collection achieved by these tech-niques have to be weighed against the extra costs incurred.

42.8 Swimming pool or large volumewater heating

Swimming pool heating presents an ideal application forsolar heating since the peak usage, particularly fo r outdoor

pools, coincides with the peak availability of sunshine.

Combining solar heating with air to air heat pumpscan provide al l year heating at very reasonable costs, (seechapter on Heat Pumps).

Unlike domestic hot w ater, where high temperatures ofabout 6O

0C are required, swimming pools need only be

heated to temperatures which are a few degrees above theair temperature. See Figure 42.9.

Typically one evacuated tube array of 3m2 will be

needed per 10 000 litres of pool volume, about one quarter

of the surface area of the pool.Since the major heat loss from a swimming pool is

by evaporation from the surface, a pool cover should belaid on the surface whenever the pool is not in use. Forthe safety of the swimmers the cover should always beremoved before swimming begins.

Circulation may be achieved by using the pump in thefiltration circuit or by using a separate pump. In winterthe solar circuit should be turned off and drained downto prevent the water in the collector from freezing an dbreaking the collectors.

Typically one evacuated tube array of 3m2

will be

needed pe r 10000 litres of pool volume, about one quarterof the surface area of the pool.

Since the major heat loss from a swimming pool isby evaporation from the surface, a pool cover should belaid on the surface whenever the pool is not in use. Forthe safety of the swimmers the cover should always beremoved before swimming begins.

Circulation may be achieved by using the pump in thefiltration circuit or by using a separate pump. In winterthe solar circuit should be turned off and drained downto prevent the water in the collector from freezing andbreaking the collectors.

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Solar Water Heating and the Plant Engineer