SOLAR COLLECTORS AND THERMAL ENERGY STORAGE

By- SAURABH SINGH

Scholar No: 152118214M.Tech

Contents

IntroductionOperating Principle of a Solar CellTypes of solar thermal collectorsNon-concentrating collectors Concentrating collectorsSolar Thermal Energy StorageMaterials of different storage systemReferences

Solar Thermal Collector

• A solar collector is a special energy exchanger which converts solar irradiation energy either to the thermal energy or Electric energy.

In Solar Thermal :

Solar irradiation Thermal Energy

In Photo Voltaic:

Solar irradiation Electrical Energy

Converted into

Converted into

Operating Principle of a Solar Cell

Types of Solar thermal collectors• 1. Non-concentrating collectors (for low temperature applications)• 2. Concentrating collectors (for high temperature applications)

Non-concentrating collectors

• Flat-plate collectors:Permanently fixed in positionIt consists of : glazing covers,

absorber plates,insulation layers, recuperating tubes

• To improve the thermal performance of a collector, heat loss from the absorber also needs to be reduced.

• The heat absorbed by the absorber plate needs to be transferred to working fluids rapidly to prevent system overheating.

• A Double-Pass structure for solar receiver, achieved a better heat transfer rate.

Evacuated tube collectors• Greater efficiency

than flat plate.• Less heat loss.

Hybrid PVT collectors

• Simultaneously convert solar energy into electricity and heat.• Cooling of PV module.• Better electrical performance.• Collection and utilization of waste heat.• Working Fluid : Water or Air.

Enhanced Hybrid PVT Collectors – Bifacial PVT

• Approx. 40% more electric energy than a conventional PVT system, • No noticeable increase in the system cost.• Waste heat recovery.• Grounds covered by snow.• Elevation of module to avoid self-shading.

Concentrating collectors

• Concentrating collectors have much higher concentration ratio than non concentrating collectors and usually equipped with sun-trackingtechniques.

Heliostat field collectors

It consists of a number of flat mirrors/heliostats.

The orientation of every individual heliostats controlled by an automatic control system.

Use of a steam generator.

Parabolic Trough collectors• Trough can focus sunlight at 30-60

times its normal intensity on the receiver tube.

• Two-axis tracking • Errors arises during the

construction and operation of it: Misalignment of the receiver. Misalignment of the reflector. Tracking errors. Parabolic profile errors. Sun image width error.

Solar Thermal Energy Storage• Main aspects that need to be considered in the design of a solar

thermal energy storage system:1. Technical properties2. Cost effectiveness 3. Environmental impact

• Technical properties:-Excellent technical properties.High thermal storage capacity is essential to reduce the system

volume and increase the system efficiency.Heat transfer rateGood stability of storage material

Cost effectiveness:-• It determines the payoff period of the investment.

• The cost of a storage system mainly consists of three parts1. Storage material, 2. Heat exchanger3. Land cost.

Materials:1. Sensible heat storage materials, 2. Latent heat storage materials 3. Chemical heat storage materials

• Sensible Heat Storage Materials• Most commonly-used solid state materials, sand-rock minerals, concrete,

fire bricks and ferroalloy materials. (Table-1)• These materials have working temperatures from 200 C to 1200 C• Excellent thermal conductive.• Low-cost materials.• Low heat capacities makes storage unit unrealistically large.

• Liquid-state materials are shown in Table 2 • Oils have rather high vapour pressure which causes

serious safety issues. • Requiring an airtight system.

• Molten salts are regarded as the ideal materials for use in solar power because of their excellent thermal stability under high temperatures, low vapour pressure, low viscosity, high thermal conductivities, non-flammability and non-toxicity

• Latent heat storage materials• Phase-transition enthalpy of PCMs are usually much higher (100-200 times,

shown in Table 3) than sensible heat, latent heat storage • Much higher storage density.• Can work in a nearly isothermal way, due to the phase change mechanism.• This makes it favorable for those applications which require strict working

Temperatures.• Low thermal conductivities

• Chemical heat storage materials• The chemical bond is capable of storing enormous quantities of energy

which can be released through detonation (explosives), controlled “burning” (rocket engines) or at slow rates such as in batteries and fuel cells

• It has storage densities in the order of GJ/m3.

The main characteristics of chemical heat storage materials:Stable long-term energy storage,Energy can be released over a wide time scale,Single stage conversion to electrical energy possible,High energy density.

Chemical Double Layer Capacitors

• In 1887, Helmholtz discovered that a liquid electrolyte-metallic conductor interface formed a boundary layer capable of storing electrical energy.

• It has the following distinguishing characteristics:1. Low single unit voltage,2. High capacitance/cell,3. Best suited for low frequency applications,4. High energy density in practical devices5. Long shelf life.

Electrolytic Capacitors

• These units can be made to operate at intermediate voltages-much higher than single cell CDL technology and much lower than the film oil capacitors.

• It have the following distinguishing features.:1. Moderate single unit voltages (>lOOV),2. Have a tendency to be unipolar,3. Moderate values of capacitance/unit,4. Reasonable frequency response,5. Capable of practical energy storage density greater than 2

KJ/kg,6. Long shelf life.

References[1] De Winter F. Solar collectors, energy storage, and materials. Massachusetts: The MIT press; 1991[2] Patented Twintex® Hybrid PV IT Collector:Level of Performances and Comparison with Thermal Solar Collector by C. Cristofari, J. L. Canaletti and G. Notton.[3] A Research on Parabolic Trough Solar Collector System Control based on Hedge Algebra[4] Comparative Analysis of Energy Storage Media and Techniques by M.F. Rose, S.A. Merryman and C.R. Johnson Space Power Institute Auburn University, AL 36849[5] A review of solar collectors and thermal energy storage in solar thermal Applications by Y. Tian a, C.Y. Zhao[6] http://www.sciencedirect.com/science/article/pii/S0306261912008549

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