Lecture Note

1. Solar Thermal Engineering
6 – Application of Solar Thermal Energy
Compiled by Solomon T/mariam
Energy Center
Addis Ababa Institute of technology
June 2020
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2. 6.1 ACTIVE AND PASSIVE HEATING
 Passive solar heating methods
 Not using active means such as pumps, fans to
collect and transfer solar thermal energy
 Passive solar heating methods utilize natural
means such as:
• radiation, natural convection, thermosyphon flow and
thermal properties to facilitate the energy
transportation.
 Active solar heating methods,
 Use active means such as pumps and fans to
collect and transfer solar thermal energy
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3. Solar water heating systems
 types of water heating system are known
 small systems for domestic hot water supply and larger
systems for industrial process heat applications. Two
types:
i. Two Natural circulation or Passive solar water heating
system (thermosyphon)
ii. Forced Circulation or active solar water heating system
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4. Passive Space Heating Systems
 Passive solar homes range from those heated
almost entirely by the sun to those with south-
facing windows that provide some fraction of
the heating load.
 The difference between a passive solar home
and a conventional home is design. And the
key is designing a passive solar home to best
take advantage of the local climate.
 Elements of design include window location
and glazing type, insulation, air sealing,
thermal mass, shading, and sometimes
auxiliary heat. 4

5.  Good passive design uses natural heat from the sun and
natural night-time cooling to keep the building at a
comfortable temperature year round.
 This can eliminate or reduce the need for expensive mechanical
heating or cooling.
 Every passive space heating systems contain the following
five basic components.
• Collector: windows, walls and roofs.
• Storage: walls and floors, large interior masses (often integrated with the
collector absorption function).
• Distribution system: radiation, free convection, simple circulation fans.
• Controls: moveable window insulation, vents both to other inside spaces or to
ambient,
• Backup system: any non solar heating system.
 By nature, passive heating is intimately concerned with
architecture, as the building itself functions as collector and
storage unit.
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6. Types of Passive Heating Systems
There are three basic types of passive
solar design based on how the five
elements are incorporated
i. Direct gain
sunlight enters the heating space and is converted
to heat at absorbing surfaces. Lastly, the heat is
distributed
ii. Indirect gain
sunlight is absorbed and stored by a mass
interposed between the glazing and the
conditioned space.
iii. Isolated gain
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7. Solar Cooking
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8. Solar Distillation
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9. Solar Drying
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10. 6.2 Solar Cooling
 Vapour compression
Refrigeration system
 Vapour Absorption refrigerating
System
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11. Principle of operation
 Ammonia vapor leaves the evaporator and enters the
absorber, where it dissolves and reacts with water to form
NH3 · H2O. This is an exothermic reaction; thus heat is
released during this process.
 The amount of NH3 that can be dissolved in H2O is inversely
proportional to the temperature. Therefore, it is necessary to
cool the absorber to maintain its temperature as low as
possible, hence to maximize the amount of NH3 dissolved in
water. The liquid NH3 + H2O solution, which is rich in NH3,
is then pumped to the generator. Heat is transferred to the
solution from a source to vaporize some of the solution. The
vapor, which is rich in NH3, passes through a rectifier, which
separates the water and returns it to the generator.
 The high-pressure pure NH3 vapor then continues its journey
through the rest of the cycle. 11

12. Cooling Requirements for Buildings
 The cooling load of a building is the rate at which heat must
be removed to maintain the air in a building at a given
temperature and humidity. It is usually calculated on the
basis of the peak load expected during the cooling season.
 For a given building the cooling load depends primarily on:
 Inside and outside dry-bulb temperatures and relative
humidities,
Solar radiation heat load and wind speed,
Infiltration and ventilation, and
Internal heat sources.

13. Cooling Using Solar Chimney
 Examples:
 Cooling of solar cells by chimney-induced
natural draft of air (Ashwin Date, Randeep Singh, Abhijit Date, Aliakbar
Akbarzadeh RMIT University)
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14. The steps in calculating the cooling load of a building are as
follows.
1. Specify the building characteristics: wall area, type of
construction, and surface characteristics; roof area, type of
construction, and surface characteristics; window area and glass
type; and building location and orientation.
2. Specify the outside and inside wet-and dry-bulb temperatures.
3. Specify the solar heat load and wind speed.
4. Calculate building cooling load resulting from the following: heat
transfer through windows; heat transfer through walls; heat
transfer through roof; sensible and latent heat gains resulting
from infiltration and ventilation, sensible and latent heat gains
(water vapor) from internal sources, such as people, lights,
cooking etc.

15.  The following equations may be used to calculate the
various cooling load for a building. Cooling loads resulting
from lights, building occupants, etc. may be estimated .
 For unshaded or partially shaded windows, the load is :
For shaded windows, the load (neglecting sky diffuse and
reflected radiation ) is:
(6.1)
(6.2)
(6.3)
For unshaded walls, the load is:

16. (6.4)
 For shaded walls, the load (neglecting sky diffuse and reflected
radiation ) is:
 For the roof the load is:
(6.5)
 Sensible-cooling load due to infiltration and ventilation is:
(6.6)
 Latent load due to infiltration and ventilation is:
(6.7)

19. Example
Determine the cooling load for a building in
Phoenix,AZ, with the specification tabulated in table
below.

21. Solution
To determine the cooling load for the building just described,
calculate the following factors in order listed.
1. Incidence angle for the south wall i at solar noon
2. Solar altitude at solar noon
3. South facing window load from Eq(6.1) at solar noon

22. 4. Shaded window load from Eq(6.2)
5. South facing wall load from Eq(6.3)
6. Shaded wall load from Eq(6.4)
7. Roof load from Eq(6.5)
8. Latent heat load (30% of sensible wall load)

23. 9. Infiltration load
10. Total cooling load described in the example

24. 6.3 Soar Industry Process Heat
 Solar energy for thermal applications in industries has proved
to be economically viable at present for temperatures less than
100 oC.
 Very large amounts of energy are used for low-temperature
process heat in industry, for such diverse applications as drying
of lumber or food, cleaning in food processing, extraction
operation in metallurgical or chemical processing, cooking,
paint drying and many others.
 In the present energy context, it is desirable to provide thermal
energy below 300 oC from solar.
 This is optional since high quality fuel, such as coal or oil, with
high flame temperature when used for low temperature
applications result in very low efficiency. Therefore, the choice
of solar energy for direct thermal applications up to 300 oC in
industries is quite obvious.
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25.  The application of solar energy for industrial
purposes may be considered in three general
categories, according to the temperature range
within which the heat is supplied.
1. Low temperature (below 100oC)
 are based on the use of flat-plate collectors, with
either air or water as the heat transport medium.
 The general principles are the same as for space
and water heating. The hot water may be made for
storage of excess heat in water or in rocks or
gravel.
 Example: drying grain, fruits, onion, solar
distillation of water. 25

26. 2.Intermediate temperatures 100 to 175oC
 Food processing, textile, laundry, and other
industries often require both hot water and low
pressure steam.
 For such applications, water can first be heated in
flat plate collectors followed by an array of
parabolic trough concentrators. In this way, water
under moderate pressure, can be heated to
temperatures above 100oC.
 Examples: Laundries, Fabric drying, textile
dyeing, food processing and can washing, paper
industries, laminating and drying glass fiber etc.
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27.  4.High temperature applications (above 175 oC)
 Steam at temperatures above 175 oC is used extensively
in industries particularly in the generation of electric
power.. The same general methods for producing high
temperature steam with the aid of solar energy are
applicable in all cases.
 Two primary questions to be considered in a possible
industrial application are:
• The use to which the energy is to be put and
• The temperature at which heat is to be delivered.
 In many industrial applications, the energy may be
needed at particular temperature or over a range of
temperatures.

28.  If for example, a low pressure steam is condensed in an
indirect dryer, the condensate will probably be re-circulated
and the solar process system will be called on to deliver
essentially all of the energy at a constant temperature level.
 Storage would usually be used in industrial processes,
except where the maximum rate at which the solar energy
system can deliver energy is not appreciably larger than the
rate at which the process uses energy. In these cases the
annual fraction of the energy needs delivered by solar
energy will be small if the process operates in other than
daylight hours.
 The investments in industrial processes are generally large,
and the transient and intermittent characteristics of solar
energy supply are so unique that the study of options in
solar industrial applications can be done by simulation
methods at costs that are very small compared to the
investments.

29. Mechanical Design Considerations
 Many industrial processes use large amounts of energy
in small spaces. If solar is to be considered for these
applications, the location of collectors can be a
problem.
 It may be necessary to locate collector arrays on
adjacent building or grounds, resulting in long runs of
pipe or duct. Collector area may be limited by building
roof area orientation.
 Existing building are generally not designed or oriented
to accommodate arrays of collectors, and in many cases
structures to support collector arrays must be added to
the existing structures.
 New buildings can be readily designed, often at little or
no incremental cost, to allow for collector mounting and
access.

30. 6.4 Solar Thermal Power Plants
 The process for conversion of solar to mechanical and
electrical energy by thermal means are fundamentally
similar to other thermal processes, and the principles
treated on radiation, collectors, storage, and systems
form the basis for estimating the performance of solar
thermal power systems.
There are two basic approaches to solar electric power
generation.
a) photovoltaic process- using solid state devices
b) Sunlight to heat and then heat to mechanical energy by
thermodynamic power cycle and finally convert the
mechanical energy to electricity (Solar thermal power)
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32. 32

33. 7.2 Thermal Power Cycles
 The process for conversion of solar to mechanical and
electrical energy by thermal means are fundamentally
similar to other thermal processes, and the principles
treated on radiation, collectors, storage, and systems
form the basis for estimating the performance of solar
thermal power systems.
 There are two basic approaches to solar electric power
generation.
 photovoltaic process- using solid state devices
 Sunlight to heat and then heat to mechanical energy by
thermodynamic power cycle and finally convert the
mechanical energy to electricity (Solar thermal power)

34.  Most of the thermal power production in the world is
based on Rankine cycle and to a smaller extent
Brayton cycle.
 Both of these are applicable to solar thermal power
conversion, with Rankine Cycle being the most
popular.
 Stirling cycle has also shown great potential and
solar thermal power systems based on this cycle are
under development.
 Most of the existing thermal power plants are based
on the Rankine cycle.
 Major components of a Rankine power plant include
boiler, turbine, condenser, pumps (condensate pump,
feedwater booster, boiler feed pump), and heat
exchangers.

35.  All of the components of a solar thermal power plant are the
same as those in a conventional thermal power plant except
the boiler.
 the boiler in a solar thermal power plant includes a solar
collection system, a storage system, an auxiliary fuel heater
and heat exchangers.
Schematic of a Solar Boiler

36.  The maximum temperature from the solar system
depends on the type of solar collection system (parabolic
trough collectors, central receiver with heliostat field,
parabolic dishes, etc).
 If the temperature of the fluid from the solar
system/storage is less than the required temperature for
the turbine, the auxiliary fuel is used to boost the
temperature. A fossil fuel or a biomass fuel may be used
as the auxiliary fuel.
Parabolic trough based power plant
 These are the most fully developed of the solar-thermal
technologies and major installations for both process heat
and electric power production exist.

37.  A parabolic trough is a linear solar collector, with a
parabolic cross-section. Its reflective surface
concentrates sunlight onto a receiver tube located
along the trough’s focal line.
 A fluid flowing through the collector is heated and
then transported through a well insulated pipe network.
 Although trough systems typically operate at temperatures
(100 to 400°C) that are significantly lower than those
achieved by other solar systems, the trough design offers
the advantage of flexibility and commercial experience.
 Moreover, because each module is connected to other
modules via various series- and parallel-flow circuits, the
system can be adjusted for a range of operating
characteristics.

38.  A solar electricity generating plant using parabolic trough
arrangement is shown in fig below.
Flow of heat transfer fluid through a SEGS

39.  All of the plants of this type use natural gas as the
auxiliary fuel so that, on average, 75 percent of the
energy is supplied from the sun and 25 percent from
natural gas.
 With power plant electrical conversion efficiencies of
the order of 40 percent and the solar field efficiencies
of 40 to 50 percent, overall efficiencies for solar to
electricity conversion of order of 15 percent are
being achieved in these plant.
 The plants can supply peaking power, using all solar
energy, all natural gas, or a combination of the tow,
regardless of time or weather.

40. Central Receiver Systems
 In this systems the incoming solar radiation is focused to a
central receiver or a boiler mounted on a tall tower using
thousands of plane reflectors, called heliostats.
The Arrangement of a Central Receiver Solar Thermal System

41.  The mirrors are installed on the ground are oriented
so as to reflect the direct beam radiation into an
absorber or receiver (boiler) which is mounted on
the top of a tower located near the centre of the field
of mirrors to produce high temperature.
 This makes it possible to position the boiler in the
field of view of all mirrors, at all hours of the day.
 Beam radiation incident on the boiler absorbed by
black pipes in which working fluid circulates and is
heated.
 The working fluid is allowed to drive a turbine and
produce mechanical energy. The turbine which is
coupled to an alternator produces electrical energy.

42. A suitable heat storage is also provided to supply heat
energy during the periods of cloudiness.
Central Receiver type Solar thermal power System

43. Figure : A picture of Solar Two, the prototype power tower. Some 1,926 heliostat mirrors are
used to produce 10 million watts of electricity.

44. Parabolic dish Systems
 This is a point focus collector that tracks the sun in two
axes, concentrating solar energy onto a receiver located at
the focal point.
Parabolic dish System

45.  Parabolic dish systems consist of modular collector
and receiver units that can either function
independently or as part of a larger system of dishes.
 Two distinct parabolic dish systems have been
developed. One method collects thermal energy
from each dish and transports it to a centralized
power conversion system; the other uses small
engines coupled to generators to provide electricity
at each dish.
 Although the engine-generator parabolic dish system
contains no thermal storage, it can be hybridized to
operate with fossil fuel during periods without
sunshine.

46.  Sterling engines seems to be the most convenient
energy conversion technology for small scale
systems (~ 25kW) because of their size, high
efficiency and potential high reliability.
 The cycle consists two isothermal and two
constant volume processes.

47. Solar Chimney Power Plant
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48. 6.5 Solar Ponds
(Evaporative Processes)
 Solar pond systems are based on a totally different
kind of thermal technology: they rely on a salt
gradient to collect and store low temperature heat.
 The salinity of the water increases with depth,
resulting in a corresponding increase in density. The
density gradient inhibits convective heat loss to the
surface, thereby enabling the pond to store solar
energy.
 salt-gradient solar pond has three distinct layers:
 The top layer, which is convective, is a few centimeters
thick and is maintained at low salinity (approximately
that of sea water). 48

49.  The central gradient layer is stable and nonconvective. Most
ponds have gradient layer thickness of about 1 meter.
 The bottom layer, where the collected energy is stored, has
as high a salinity as possible and is convective. The
temperature at the bottom of the solar pond can reach 90°C,
which is sufficient to drive the vapor generator of an organic
Rankine-cycle engine.
 In addition, because solar ponds have a rather large
storage capacity, they can store thermal energy
seasonally.
 The potential of solar ponds to provide freshwater,
process heat and electricity appear promising but only
very few small scale applications exist to date.
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50. Solar Pond Electric Generating Element