Solar Thermal Energy Conversion

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 Utilization of Solar Energy

 Solar energy is mainly utilized by means of two technologies, namely-

(a) solar thermal energy conversion and (b) solar photovoltaic (PV)
energy conversion.

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 Solar Thermal Energy Conversion

 Heat transferring fluid carries heat from the solar collectors to heat exchangers
and heat storage tanks in solar water heating systems.

Source: https://energy.gov/energysaver/heat-transfer-fluids-solar-water-heating-

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 Solar Thermal Energy Conversion

Selection of heat transferring fluid is based on following criteria-

 Coefficient of expansion – the fractional change in length (or sometimes in volume,

when specified) of a material for a unit change in temperature.
 Viscosity – resistance of a liquid to shear forces (and hence to flow).
 Thermal capacity – the ability of matter to store heat.
 Freezing point – the temperature below which a liquid turns into a solid
 Boiling point – the temperature at which a liquid boils.
 Flash point – the lowest temperature at which the vapor above a liquid can be ignited
in air.
 Corrosiveness and Stability.

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 Solar Thermal Energy Conversion
Some heat exchanging fluids are-
 Air: Air will not freeze or boil, and is non-corrosive. However, it has a very low heat capacity, and tends
to leak out of collectors, ducts, and dampers.
 Water: Water is nontoxic and inexpensive. With a high specific heat, and a very low viscosity, it's
easy to pump. Unfortunately, water has a relatively low boiling point and a high freezing point.
 Glycol/Water Mixtures: Glycol/water mixtures (glycol) are "antifreezes." These mixtures provide
effective freeze protection as long as the proper antifreeze concentration is maintained.
 Hydrocarbon Oils: Hydrocarbon oils have a higher viscosity and lower specific heat than water.
They require more energy to pump. These oils are relatively inexpensive and have a low freezing
point.
 Refrigerants/Phase Change Fluids: Chlorofluorocarbon (CFC) refrigerants, such as Freon,
were the primary fluids used by refrigerator, air-conditioner, and heat pump manufacturers because
they are nonflammable, low in toxicity, stable, noncorrosive, and do not freeze.

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 Solar Thermal Energy Conversion

Performance indices:
 Collector Efficiency: It is defined as the ratio of the energy actually absorbed and
transferred to the heat transferring-fluid by the collector (useful energy) to the energy
incident.

 Concentration Ratio: Ratio of the area of aperture of the system to the area of the receiver.

 Temperature Ranges: The range of the temperature to which the heat transferring fluid is
heated up by the collector.

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 Types of Solar Collectors

1. Non – Concentrating type (Flat-plate collectors):

The absorbing surface is approximately as large as the overall collector area which intercepts
the sun rays.

2. Concentrating type collectors:

Large areas of mirrors or lenses focus the sun light onto a smaller absorber.

Cross section of typical liquid flat plate Concentrating solar collector

collector

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Types of Solar Collectors

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 Non-concentrating Type (Flat plate Collectors)

The basic elements of these collectors are:

 Transparent Cover: May be layers of glass or plastics.

 Blackened Absorber Plate: See Table-I, below.

 Tubes, Channel or Passages: Copper tubes, steel.

 Weather tight, insulated container: Glass wool across wall.

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Non-concentrating Type (Flat plate Collectors)

Table-I: Materials for absorber plate

Break down
Absorptance Emittance
Material temparature
(α) (e) (°C)
Black silicon
paint 0.86-0.94 0.83-0.89 350
Black Copper
over Copper 0.85-0.9 0.08-0.12 450
Black chrome
0.92-0.94 0.07-0.12 450
over nickel

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 Flat Collector Performance and Efficiency
Effects of various parameters on performance:
 Selective surface: Absorber plate surface which have high absorptivity to
incoming radiations and low emissivity.
 Number of covers: With increase in the number, the flux reaching absorber
surface reduces. But almost 2 are required for high efficiency.
 Spacing: Spacing between two covers or between cover and absorber should be
such that the losses are minimum. (Usually 4-8cm).
 Collector tilt: Tracking the sun is important. Therefore tilting of surface is
required.
 Dust on top of cover: The deposition of dust reduces the transmission of flux
from cover to the absorber. Therefore, regular cleaning is required.

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 Concentrating Type Collectors
1) Non-Focus Type
a) Modified Flat-Plate Collector
b) Compound Parabolic Concentrating (CPC) Collectors

2) Focus Type
a) Cylindrical Parabolic concentrator
b) Fixed mirror solar concentrator One Axis Tracking
c) Linear Fresnel Lens collector
d) Paraboloidal dish collector
e) Hemispherical bowl mirror conc. Two Axis Tracking
f) Circular Fresnel Lens conc.
g) Central Tower Receiver
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Concentrating Type Collectors

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Concentrating Type Collectors

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Concentrating Type Collectors

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Comparison of Concentrating Type and Non-Concentrating
Type Collectors

 In concentrating type solar collectors, solar radiations are converged from a large area into
smaller area using optical means (reflection/refraction).
 Direct radiation moving in a straight line can be reflected/refracted while diffused
radiation doesn’t follow optical rule.
 Diffuse component can’t be concentrated.
 Concentrating type absorbs only beam while non- concentrating absorbs both types of rays
(advantage).
 Flat plat collector, simple in construction and doesn’t require sun tracking and can be
installed on rigid platform.
 Flat plat collectors require less maintenance and can withstand harsh outdoor conditions
compared to concentration type collectors.
 Absence of optical concentration in flat plate collectors is a principal advantage.
 High temperatures can’t be obtained in non-concentration type collectors.
 High temperature can be attained by concentration type collectors.
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 Solar thermal Energy System

 Solar thermal systems use the energy provided by the sun to create heat.

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Solar thermal Energy System

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Applications of Solar Thermal Energy Systems

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 Applications of Non- Concentrating (Flat Plate)
Collectors

1. Solar Water Heater:

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 Solar Water Heater
Solar Water Heater:
 Water is used as heat transfer fluid.
 Thermally insulated storage tank is mounted above the collector.
 The heated water of the collector rises up and equally replaces the amount of
cold water.
 Cycle repeats until all water is heated up.
 Thermo-siphon action naturally circulates the water.
 This is known as passive heating scheme as no pumps are required.
 Heats water at around 60-80° Celsius.
 Life span of about 10-12 years.

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 Solar (Passive) Heating
2. Solar Passive Heating:

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 Solar (Passive) Heating
Solar (Passive) Heating:
 These systems are used to maintain temperature for comfort.
 Air is used as a heat transportation medium.
 The specially developed buildings are called solar houses.
 A thick Trombe Wall, is made of concrete, stones and composites brick blocks
used as thermal storage.
 Its outer side is painted black for increased absorption.
 The circulation of hot air continues until all air is heated up.
 Heating can be adjusted using inlet and outlet vents.
 Dampers may be used at top to remove access air between glazing and Trombe
wall.

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 Solar (Passive) Heating

Solar (Passive) Heating:

 Reflective horizontal surfaces are used.
 It provides additional radiation to thermal storage.
 Rock bed storage is also used to store energy.
 Keeps space warm during nights.

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 Solar Passive Cooling
3. Solar (passive) Cooling:
 Reduction in thermal loads-
a) Reduction in direct sunlight entering the building (Shading etc.)
b) Reduction in conduction of heat through walls (by insulation).
c) Proper sealing should be provided from outside warm air.

 There are various methods of space cooling-

a) Shading.
b) Ventilation.
c) Evaporation.
d) Radiation.
e) Ground Coupling.
f) Dehumidification.

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 Solar Passive Cooling
Solar (passive) Cooling:
 Shading method prevents heating from direct sun light entering the house through
reflective roofs and shades.
 In Ventilation method, the warm air is driven out by Chimney effect.

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 Solar Passive Cooling
Solar (passive) Cooling:
 Evaporation is similar to ventilation, difference is at inlet a saturated filter is
used which increase the relative humidity.

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 Solar Passive Cooling
Solar (passive) Cooling:
 In Earth Coupling method, the temperature of the soil acts as a heat sink
to cool a building through convection. This is effective in hot climates.

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 Solar Passive Cooling
Solar (passive) Cooling:

 In Dehumidification system, the moisture content is reduced and cooling is

maintained by adsorption and evaporation.
 Solid adsorbent material (silica gel, CaCl) and water baths are provided.

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 Solar Industrial Heating Systems
4. Solar Industrial Heating Systems:
 These systems are active systems using some pumps or blowers to circulate air or
water.

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 Solar Industrial Heating Systems
Solar Industrial Heating Systems:
 These systems are active systems using some pumps or blowers to circulate
air or water.

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 Solar Refrigeration and Air Conditioning Systems

5. Solar Refrigeration and Air-Conditioning System:

 LiBr and Water Absorption Refrigeration cycle.

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 
Other Applications
6. Solar Furnace:
 Ideal tool to study of chemical, optical, electrical and thermal properties of a
specimen/ materials at high temperature.
 It has a concentrator and large no. of small heliostats (with sun tracking).
 Temp of about 3500OC obtained.

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Useful References/Resources:
[1] B. H. Khan, “Conventional Energy Source” Second Edition, Tata
McGraw Hill, 2009.
[2] https://nptel.ac.in/courses/121/106/121106014/

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