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Module 1: Solar and Wind Energy

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Solar Energy Conversion

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Hi friends, now we will discuss on the topic renewable energy that is solar energy productionpart 2.The content of this discussion is the techniques for solar energy conversion to usable form,that is solar thermal and solar photovoltaic.So if you see the solar energy can be used into different forms into different routes.So solar energy is converted to organic food, organic materials through the plants, throughphotosynthesis.Solar energy can be used to produce hydrogen, photocatalysis routes.Solar energy can be converted to electricity through photovoltaic routes.Solar energy can be converted to heating through thermal route and then that heat can be usedfurther for the production of electricity.So there are a number of ways or the routes through which the solar energy can be convertedto usable form.So this is a very complex process, still the simulation of this process is not possible,this is under development, that is the hydrogen production through photolysis, this is alreadydeveloped, this photovoltaic is also developed.So, we will discuss this thermal route and photovoltaic routes for the production ofelectricity by this solar energy.So we have thermal conversion and we have PV conversion, basically we will be concentratingon these two routes.Now see already passive design of building helps us to get solar energy for heating applications,that is not actively or directly using some liquid or something to capture the energyand then like this, that is that building is designed in such a way that we are ableto get some amount of energy automatically by that way, but by using some heating devicealso, we can collect more energy for this application.Here at the rooftop, we can use some heating device that is collector that will collectthe heat.Then we can send the liquid or that is water here cold water, it will pass through it,the sunlight is coming here and the water is heated and it is coming to this, and thenthere is some heat transfer, so then that way we can get the energy collected at thetop is available in the room and then we can exchange this energy here also.So, here we have used some external device which is collecting the solar radiation andhelping to heat the liquid pass through this particularly designed device so that is calledsolar thermal technology and where the thermal collectors for water heating, space heatingand other uses we can use.So we can send here the water or we can send here air also, if we send air, then also airwill be heated and the room will also be heated, and if we use the water, then also the waterwill be heated and then can be used for other application, but this technology normallythe temperature of water does not rise very high, then the temperature of water maybe60-70 like this, 70 degree centigrade like this.So, solar thermal collector, which we are using at the top, that may be of differenttypes.It may be flat plate collector, it may be evacuated tube collectors, or it may be aconcentrating collectors.So there are basically 3 types of collectors so far has been reported that is a flat plate,evacuated tube, and then concentrating collectors.So, we will be discussing these one by one.The main uses are water heating and space heating for homes and business establishmentby this technology.But when we use concentrating collectors, then we can get very high temperature wateror even can produce high temperature steam and that can be used for the production ofelectricity in steam turbine.Now, we will see the flat plate collectors.A flat plate collector consist basically an insulated metal box with the glass or plasticcover and then a dark-colored absorber plates, so this is the construction of different partsof a flat plate collectors.Solar radiation is absorbed by the absorber plate and transferred to the fluid that circulatesthrough the collector in tubes and in air dry collector the circulating fluid is airwhereas water can be used also as a fluid, and in some cases some anti-freezing agentis also used, otherwise if the temperature is lower inside the room below say 0 degreecentigrade, it will be frozen, so that anti-freezing agent is also used to capture the sunlightwhen the temperature is very very less it is say below 0 degrees centigrade.Here we see the flat plate collectors, here water is coming here, so we have your absorberplate, then sunlight is coming here and then absorber plates will absorb it and it willbe heated up and then through which the tubes the water is going through so that will beheated and will be coming out.So, this is a very simple design of this, primitive design we can say, that is it isa box type arrangement.Then from the top, the sun is coming, sunlight is coming and then from this we are havingthe water flow or air flow and we are getting here the heated water, but this method asI have mentioned that 30 to 70 degree centigrade temperature rise can be available in thiscase, we cannot get very high pressures steam that is the flat plate collectors, and basicallythese are used in home applications like residential hot water heating.Then evacuated tube solar thermal design came later.These collectors use a thermos bottle type arrangement or collector.So in this case, the vacuum is created so that the vacuum due to the availability ofvacuum it is able to give the high temperature water, so the temperature of the outlet ofthis collector is more than that of the flat plate collector and the vacuum acts as insulatorand reducing any heat loss significantly to the surrounding atmosphere either throughconvections or radiations making the collector much more efficient than the initial insinuatingthat flat plate collectors have to offer.So this is the features of this evacuated tube solar thermal collectors.Then we are coming to concentrating collectors.So this is the latest development of this collector, that is sunrays after incidentthose are directed in such a way that will be focused on a certain point or in a centralline.So the concentration of radiation will be higher at the receiver.So here we have two things, one is your concentrators and another is your receiver.So concentrators or collectors, so those are initially collecting the sunrays and thenit is focusing these rays to a focal point, on a certain point that is the receiver.So one is collectors and another is receivers.So depending upon the shapes of these collectors and receivers, the extent of concentrationalso varies which is defined by the optical concentration ratio that is average flux overthe receiver divided by flux over the aperture of the collectors, that is how much insolationis coming on the collector and then how much insolation we are getting at the receiver.So receiver insolation divided by the collector insolation that is we can say optical concentrationratio.As you see here for this, we need some mechanical help, that mechanical design will play a role,we may we may need to change the orientation of the collector or we have to equip it insuch a way that maximum concentration is possible.So here we see the plane reflector plane receiver if we use, then the concentration ratio canbe 1 to 4, but if we use the conical reflector and then cylindrical receiver, then you canget 4 to 10, if we use parabolic cylindrical reflector and cylindrical receiver, then youcan get 10 to 100.When it is paraboloidal reflector and spherical receiver that will give us 10,000 up to.So that is the maximum capacity or the maximum concentration we can get by this type of arrangement.So we see here the different types of concentrating collectors, one is your parabolic trough system,so this is trough systems, like this type of structure it is the trough.So it will be if we have a tube here so water, rays will be coming here and these after reflectionit is going and concentrating on this, so this is our receiver, so this is a parabolictrough.So it may be a parabolic dish, so here the parabolic surface, rays are coming, then incident,after its reflection it is going and then it is it is focusing on a point.So there are some mirrors, here also some mirrors are there.So rays are coming and it is being reflected and it is being concentrated.So here it is, this rod is there, here the point is there for the concentration, anda receiver for the collecting of the concentrated rays.Here, we may have the power tower, so power tower, so we have mirror at the bottom, soray will come and it will be reflected and will be focused on the point at the top, sothis is called power tower.So these are 3 major important designs for the collection of the sunrays by these concentratingcollectors.Apart from these, we have stationary concentrating collector.So stationary concentrating collectors use compound parabolic reflectors and flat reflectorsare used for directing solar energy to an accompanying absorber or aperture througha wide acceptance angle.So as the wide acceptance angle means due to the use of wide acceptance angle, thesereflectors eliminates the need for a sun tracker, so sun tracker is not needed for this typeof the stationery concentrating collectors.These are some photographs how it looks like, so parabolic disc and parabolic trough, sohow it looks like it is shown here.So you see here, it is a disc that is after reflections it concentrates to this one pointand then here it is a tube, so it is concentrated on a tube, that is the parabolic trough.Now, we will see how can we calculate the performance of a flat plate collector, theprimitive one say, how can we determine the collector efficiency, we will discuss now.So in a flat plate, as you have seen that one flat surface is there, so where we havehere one surface and then sunray will come and it will be taken up by the plate and someamount of energy will go out.So energy in is your Ein and energy out is Eout.So Ein–Eout, so that is the energy taken up by the water or we can say that energygained by the system.So useful energy gain we can say that is equal to, Qu = Ein-Eout, that is energy which iscoming inside this that will be absorbed first, that will be collected by the collector, thenit is absorbed from the top and the transmitted towards the bottom where the liquid is flowing.So we have two terms here say collector efficiency that will influence the overall efficiencyof the process, then absorptivity of collector, how the collector is able to absorb the solarradiation that will also influence the overall performance.Then transmissivity of glass cover, so one glass cover at the top, so how it is transmittingthe light so that will also influence the overall performance and here we have insulation,here through which the water is passing through the tube we have some insulation, so therewill be some losses of heat energy through this if insulation is not proper, so thatinsulation loss is there, so overall loss coefficient is also considered which is importantand will influence on the overall performance of the system.This Tf and To, temperature of the fluid in the tubes and the ambient temperature thatwill also influence the performance.Another is important incident solar radiations on collectors, what is the radiation it iscoming on the collector that will also influence the process performance.So if we want to get the collector efficiency here, this equal to obviously how much energywe are getting that is Ein-Eout /how much energy was available in the incident rays.So if the surface area is Ac and It is incident solar radiation, so this is our total solarradiation in kilowatt per meter squared unit and this is our energy we are getting amountof heat we are getting from the system, so this by this, Qu/(Ac x It ) is our collectorefficiency.Now efficiency is equal to Qu/AcIt, but now what is our Qu, Qu is also this one that Fa,FR that is equal to collector efficiency x Ac x α τ IT – UL (Tf,o-Ta).So this is the expression for the Qu.So if we put this expression in place of Qu, so efficiency will be like this.So efficiency is equal to Qu/AcIT x (FR (τα) – FRULx (Tfo-Ta)/IT).So now this equal to y = mx+c type of expression, y = a+b type of expression.So if we plot efficiency versus this, we can measure the Tf, we can measure Tm Me and wecan measure the intensity of the light which is coming that IT.So then we can plot and we can get the value of this A and B and A and B we can get thevalue of FR we can get the UL like this.So now we can get the value of A and B.Now, solar thermal power plant.So how the solar technique, thermal technique can be used for the electricity productionthat part will discuss now.So we have some say some collector here, so we have collector, so that is a parabolictrough collector here, so these are the tubes so we are sending cold water, so it is goingthrough these tubes and it is heated up and ultimately we are getting steam.So this steam is going and this can be used for different applications.It can be used in hot tank and it can be going for the super heater and this can be usedin a turbine for the electricity production.So this is the method through which the electricity is produced.The steam is generated here, it goes through this and it is after super heating, it goesthere to the turbines, which is coupled with the generator, which gives us electricityand the exhaust steam is condensed and again used here or it can be sent here for the heatrecovery or like this So this is the arrangement for the use of heat captured by the steamin this plant.(Refer Slide Time: 18:03So one example of this power tower in Barstow, California.This figure shows that how the solar energy is produced by the capturing of solar lights.So this is 10 megawatt solar power plant in Barstow, California, that is 1900 heliostats,each 20 feet by 20 feet is used and a central 295 five feet tower.So all are focused in this tower and then it gives us that amount of electricity.Now we are coming to solar photovoltaic.So solar photovoltaic, it is another type of technology and which is the latest developmentof the solar technology and most important one, which can give us electricity directlywithout the production of steam, unlike solar thermal route.So here what happens, solar energy comes on some surface which is having an N and P typesemiconductors and then when the solar cell is P type semiconductor and N type semiconductor.The solar light hitting the cell produces negatively charged electrons and positivelycharged holes, so negatively charged electrons and positively charged holes.So sunlight is falling on it and these holes and electrons and generated and then electronsare moving towards this N-type semiconductor and P-type semiconductor, those are the positiveholes are moving towards it and they are collecting these things, and when we will add externalload, then we will get electricity through it.So we are getting the electricity now.So this method is the most important method for the conversion of electricity from thesolar rays.Now we will see different types of materials for the PV cells and their efficiency.So solar cell, it may be silicon semiconductor, it may be compound semiconductor, it may beorganic semiconductor.So silicon semiconductor, it may be crystalline or amorphous and crystalline maybe singlecrystal maybe poly crystalline, and their conversion efficiency for the single crystalline10 to 17%, for poly crystalline 10 to 13%, it may be non-crystalline that is 7 to 10%.Compound semiconductor that is gallium arsenide, it is 18 to 30% conversion efficiency.Organic semiconductor that is dye-sensitized type, it is 7 to 8% and organic thin layertype 2 to 3%.So these are the different types of materials have been investigated for the productionof electricity in a PV photovoltaic mode.Then conversion efficiency, obviously electricity output by energy of insolation on cell into100 that will be in the percentage.Now we will see the volt and current which can be achieved in this PV cell.So this is for low insolation, this is the graph people have reported and for high insolation.So 0.5 volt for silicon photovoltaic cell is available for a single cell and intensityof insolation and size of cell will also influence the amount of voltage generated.Now this is one photograph for PV cell which can give some details on this.So here, we have these are aluminum electrode and this blue colored film that is anti reflectionfilm and then black surface, this black surface is P-type conductor semiconductor and allblack surface is aluminum electrode with full reflection and this is front surface N-typeside, so N-type side and P-type side is backside, that is black side.This is your PV module, one module single crystal, and this is your polycrystalline,so this is poly crystalline, this is single crystalline.So single crystals which has more capacity 128 watt, this is 120 watt, same size bothare having, but single crystal will give some more capacity as it is evident from this.Then there is the hierarchy of the PV cells, so a single cell we see here, 2-3 watt, sothis is black and blue, so alumina and N and P type that we were talking about the N typeand P type semiconductor, so we are using it and 2-3 watt is generated, so you have0.5 volt and 5-6 amps and 2-3 watt, so about 10 cm size.Then gradually the improvement came and then module structure came, so this is our module,so this module is able to give 20-30 volt, 5-6 amps and then 100-200 watt and 1 meterits size.The number of modules was put in arrays and then we get array that is say 200-300 voltit is possible, 50 amps to 200 amps is possible, 10-50 kilowatt capacity was developed, andthe size is around 30 meter.So gradually the PV cells developed.This is the roughly size of PV power station.If we need to get say 20 kilowatt power through PV techniques, then it requires 10 meter by20 meter roof area.So 1 kilowatt PV need 10 meter square area for its production.Now the temperature also influences the performance of this process.The temperature increases, efficiency of the process decreases.As shown here say a typical 25 degree centigrade we are having, if it is an amorphous siliconPV cell, then this will be having around 8% percent efficiency.If we go to say 65 degrees centigrade which is available in some places during summerat the rooftop, then there it is slightly less than this.In this case for crystalline cell if it is around say 13% percent, so here it is comingto say 11%, so 2% down due to the increase in temperature from 25 to 65 degrees centigrade.So that way we have come to know that the efficiency of PV cell can also be changedwith increasing the temperature.So up to this in this class.Thank you very much for your patience.