RES lecture

1. SOLAR POND
The sun is the largest source of renewable
energy and this energy is abundantly
available in all parts of the earth.
It is in fact one of the best alternatives to the
non renewable sources of energy.
Solar energy has been used since prehistoric
times, but in a most primitive manner.

2. • Before 1970, some research and
development was carried out in a few
countries to exploit solar energy more
efficiently, but most of this work
remained mainly academic.
• One way to tap solar energy is through
the use of solar ponds. Solar ponds are
large-scale energy collectors with
integral heat storage for supplying
thermal energy.

3. • It can be use for various applications,
such as process heating, water
desalination, refrigeration, drying and
power generation.

4. • A solar pond is a body of water that
collects and stores solar energy.
• Solar energy will warm a body of water
(that is exposed to the sun), but the water
loses its heat unless some method is used
to trap it.
• Water warmed by the sun expands and
rises as it becomes less dense.
• Once it reaches the surface, the water
loses its heat to the air through
convection, or evaporates, taking heat
with it.

5. • The colder water, which is heavier, moves
down to replace the warm water, creating
a natural convective circulation that mixes
the water and dissipates the heat.
• The design of solar ponds reduces either
convection or evaporation in order to
store the heat collected by the pond.
• They can operate in almost any climate.
• A solar pond can store solar heat much
more efficiently than a body of water of
the same size because the salinity
gradient prevents convection currents.

6. • Solar radiation entering the pond
penetrates through to the lower layer,
which contains concentrated salt solution.
• The temperature in this layer rises since
the heat it absorbs from the sunlight is
unable to move upwards to the surface by
convection.
• Solar heat is thus stored in the lower layer
of the pond.

7. • WORKING PRINCIPLE
• The solar pond works on a very simple
principle.
• It is well-known that water or air is
heated they become lighter and rise
upward.
• Similarly, in an ordinary pond, the sun’s
rays heat the water and the heated water
from within the pond rises and reaches
the top but loses the heat into the
atmosphere.

8. • The net result is that the pond water
remains at the atmospheric temperature.
• The solar pond restricts this tendency by
dissolving salt in the bottom layer of the
pond making it too heavy to rise.
• A solar pond is an artificially constructed
water pond in which significant
temperature rises are caused in the lower
regions by preventing the occurrence of
convection currents

9. • The more specific terms salt-gradient
solar pond or non-convecting solar pond
are also used.
• The solar pond, which is actually a large
area solar collector, is a simple
technology that uses a pond between
one to four metres deep as a working
material.

10. • The solar pond possesses a thermal
storage capacity spanning the seasons.
• The surface area of the pond affects the
amount of solar energy it can collect.
• The dark surface at the bottom of the
pond increases the absorption of solar
radiation.
• Salts like magnesium chloride, sodium
chloride or sodium nitrate are dissolved in
the water, the concentration being densest
at the bottom (20% to 30%) and gradually
decreasing to almost zero at the top.

11. • Typically, a salt gradient solar pond
consists of three zones.
• An upper convective zone of clear fresh
waters that acts as solar collector/receiver
and which is relatively the most shallow in
depth and is generally close to ambient
temperature.
• A gradient which serves as the non-
convective zone which is much thicker and
occupies more than half the depth of the
pond.

12. • Salt concentration and temperature increase
with depth.
• A lower convective zone with the densest
salt concentration, serving as the heat
storage zone.
• Almost as thick as the middle non-
convective zone, salt concentration and
temperatures are nearly constant in this
zone.

13. • When solar radiation strikes the pond, most of
it is absorbed by the surface at the bottom of
the pond.
• The temperature of the dense salt layer
therefore increases.
• But the salt density difference keeps the
‘layers’ of the solar pond separate.
• The denser salt water at the bottom prevents
the heat being transferred to the top layer of
fresh water by natural convection, due to
which the temperature of the lower layer may
rise to as much as 95°C.

16. • APPLICATIONS
• Process heat
• Studies have indicated that there is excellent
scope for process heat applications (i.e.
water heated to 80 to 90°C.),
• when a large quantity of hot water is
required, such as textile processing and dairy
industries.
• Hot air for industrial uses such as drying
agricultural produce, timber, fish and
chemicals and space heating are other
possible applications

17. A visual Demonstration of how a Solar Pond is used to Generate Electricity

18. • Desalination Drinking water is a chronic
problem for many villages in India.
• In remote coastal villages where seawater
is available, solar ponds can provide a cost
effective solution to the potable drinking
water problem.
• Desalination costs in these places work
out to be 7.5 paise per litre, which
compares favourably with the current
costs incurred in the reverse osmosis or
electrodialysis/desalination process.

19. • Refrigeration applications have a
tremendous scope in a tropical country
like India.
• Perishable products like agricultural
produce and life saving drugs like
vaccines can be preserved for long
stretches of time in cold storage using
solar pond technology in conjunction
with ammonia based absorption
refrigeration system.

20. • ADVANTAGES
• Low investment costs per installed
collection area.
• Thermal storage is incorporated into the
collector and is of very low cost.
• Diffuse radiation (cloudy days) is fully
used.
• Very large surfaces can be built thus large
scale energy generation is possible.
• Expensive cleaning of large collector
surfaces in dusty areas is avoided.

21. Solar Distillation
• There is an important need for clean, pure
drinking water in many developing
countries.
• Often water sources are brackish (i.e.
contain dissolved salts) and/or contain
harmful bacteria and therefore cannot be
used for drinking.

22. • In addition, there are many coastal
locations where seawater is abundant but
potable water is not available.
• Pure water is also useful for batteries and
in hospitals or schools.
• Distillation is one of many processes that
can be used for water purification.
• This requires an energy input, as heat,
solar radiation can be the source of
energy.

23. • In this process, water is evaporated, thus
separating water vapour from dissolved
matter, which is condensed as pure water.

28. • Energy requirements for water distillation
• The energy required to evaporate water is
the latent heat of vaporisation of water.
• This has a value of 2260 kilojoules per
kilogram (kJ/kg).
• This means that to produce 1 litre (i.e. 1kg
since the density of water is 1kg/litre) of
pure water by distilling brackish water
requires a heat input of 2260kJ.

29. • This does not allow for the efficiency of the
heating method, which will be less than 100%,
or for any recovery of latent heat that is
rejected when the water vapour is condensed.
• It should be noted that, although 2260kJ/kg is
required to evaporate water, to pump a kg of
water through 20m head requires only
0.2kJ/kg.
• Distillation is therefore normally considered
only where there is no local source of fresh
water that can be easily pumped or lifted.

30. • How a simple solar still operates
• Figure shows a single-basin still.
• The main features of operation are the
same for all solar stills.
• The incident solar radiation is transmitted
through the glass cover and is absorbed as
heat by a black surface in contact with the
water to be distilled.
• The water is thus heated and gives off
water vapour.

31. • The vapour condenses on the glass
cover, which is at a lower temperature
because it is in contact with the ambient
air, and runs down into a gutter from
where it is fed to a storage tank.

32. • Design objectives for an efficient solar
still
• For high efficiency the solar still should
maintain:
• a high feed (undistilled) water
temperature
• a large temperature difference between
feed water and condensing surface
• Low vapour leakage.

33. • A high feed water temperature can be
achieved if:
• a high proportion of incoming radiation
is absorbed by the feed water as heat.
• Hence low absorption glazing and a
good radiation absorbing surface are
required
• heat losses from the floor and walls are
kept low
• the water is shallow so there is not so
much to heat.

34. • A large temperature difference can be
achieved if:
• the condensing surface absorbs little or
none of the incoming radiation
• condensing water dissipates heat which
must be removed rapidly from the
condensing surface by, for example, a
second flow of water or air, or by
condensing at night.
•

35. SOLAR DRYING
• The heat from the sun coupled with the
wind has been used to dry food crops for
preservation for several thousand years.
• Other crops such as timber need to be
dried before they can be used effectively,
in building for instance.

36. • This sun-drying has often developed
into solar-drying,
• where the drying area is in an enclosed
ventilated area – often with polythene,
• acrylic or glass covering - as a more
efficient harnessing of the elements of
the drying operation.

37. • There are innumerable designs in use
and each has its advantages and
disadvantages.
• However, there are three basic designs
upon which others are based:
• solar cabinet dryer,
• tent-dryer, and
• solar tunnel dryer.
• These are discussed below after a brief
description of the principles of drying.

38. • Basic principles of drying depend upon:
• Temperature, humidity and quantity of
air used
• Size of the pieces being dried
• Physical structure and composition
• Airflow patterns within the drying
system

39. • Heat is not the only factor which is
necessary for drying.
• The condition, quality and amount of air
being passed over and through the
pieces to be dried determine the rate of
drying.
• The amount of moisture contained in
the air to be used for drying is
important and is referred to as absolute
humidity.

40. • The term relative humidity (RH) is more
common and is the absolute humidity
divided by the maximum amount of
moisture that the air could hold when it is
saturated.
• RH is expressed as a percentage and fully-
saturated air would have an RH of 100%.
• This means that it cannot pick up any more
moisture.
• Air containing a certain quantity of water
at a low temperature will, when heated,
have a greater capacity to hold more
water.

41. • The table below gives an example of air
at 29oC with an RH of 90%.
• Such air, when heated to 50oC will then
have an RH of only 15%.
• This means that instead of only being
able to hold only an extra 0.6 grams of
water per kilogram (at 29oC), it is able to
hold 24 grams per kilogram.
• Its capacity to pick up moisture has been
increased because it has been heated.

42. The effect of air temperature upon relative humidity

43. • When placed in a current of heated air,
food initially loses moisture from the
surface.
• This is the constant rate period.
• As drying proceeds, moisture is then
removed from inside the food material,
starting near the outside.
• Moisture removal becomes more and
more difficult as the moisture has to move
further from deep inside the food to the
surface.

44. • This is the falling-rate period.
• Eventually no more moisture can be
removed and the food is in equilibrium
with the drying air.
• During the falling-rate period, the rate of
drying is largely controlled by the chemical
composition and structure of the food.
• Design of a dryer depends upon the drying
rate curve of the material to be dried but
these curves are indicative only and
depend upon the factors mentioned
above.

45. • The heat required to evaporate water is
2.26kJ/kg.
• Hence, approximately 250MJ (70kWh) of
energy are required to vaporise 100kg
water.
• If the ambient air is dry enough, no heat
input is essential.
• The greatest potential for drying crops in a
short time is when the ambient air is arid
and warm.

46. • If the air is warm then less air is needed.
This temperature will itself depend mainly
on the air temperature but also on the
amount of solar radiation received
directly by the food being dried.

47. • Solar drying Operation
• All dryers need ventilation to be able to
dry crops effectively.
• Air movement can be by natural
convection or can be assisted using fans.
• Solar food drying can be used in most
areas but how quickly the food dries is
affected by the variables indicated above,
especially the amount of sunlight and
relative humidity.

48. • Typical drying times in solar dryers are
from 1 to 3 days depending on sun,
• air movement,
• humidity and the type of food to be
dried.
• Most dryers are black inside, either
painted or with black polythene inserts
to absorb as much solar radiation as
possible.

49. • Cabinet dryers
Figure 1: The Brace solar cabinet dryer

50. Figure 2: Section through the cabinet dryer showing the flow of air in through
the vent holes in the underside past food placed on the drying trays and out of
the holes at the top of the cabinet.

51. • Tent dryers
• The distinguishing feature of tent and
cabinet driers is that the drying
chamber and the collector are
combined.
• Such dryers provide protection from
dust, dirt, rain, wind, and pests.

52. Figure 3: Tent Dryer

54. • A much smaller tent dryer is shown below.
• Very similar to a cabinet dryer, it
demonstrates the overlap in designs
between solar dryers):
Figure 4: A small solar tent dryer, Ghana. Figure 5: Large solar tent dryer, Ghana

55. • Solar tunnel dryers
• Many solar dryers employ the use of
photovoltaic cells to power fans to blow air
across the drying area. Chief among this type of
dryer is the Hohenheim dryer produced by
Innotech in Germany.
• By using a fan to create the airflow, drying time
can be reduced substantially.
• Air flows across an area usually painted black
(the collector area) to absorb the sun’s heat and
is blown across trays containing the material to
be dried. The diagram below shows the features
of the dryer.

56. Figure 6: Solar tunnel dryer layout.

57. Figure 7: A Hohenheim dryer, Ghana. Black paint being applied to the collector

58. • There are several other types of solar
dryer, many involving insulation and
different air-flow methods.
• Some have a chimney fitted to the outlet
to encourage better airflow, other make
use of external heating sources such as hot
water to allow further drying at night or
when cloud cover prevents efficient
drying.
• However, all are essentially variations on
the three described above.

59. Solar drying compared with solar drying
Figure 8: View inside a Hohenheim solar tunnel dryer

60. • The great advantage of open-air drying is
that it there is minimal capital outlay.
• It is labour-intensive, although where
labour is cheap this is not a drawback.
• An important advantage of solar drying is
that the product is protected from rain,
insects, animals and dust.

61. • This improves the hygiene and quality of
the product as well as avoiding the
covering, or transferring the crop to a
sheltered area during rain.
• Solar drying, especially when using fans,
gives some control of the drying process
at elevated temperatures, and can be
faster, which reduce the likelihood of
mould growth and spoilage of the
product.

62. • However care is needed when drying food at
too high a temperature since too rapid drying
can result in the outside of the food becoming
dry on the outside and still wet on the inside.
• This is called “case-hardening”.
• This can give a false impression that the whole
food is dry.
• On subsequent storage the trapped moisture
will migrate to the outside of the food, raising
the humidity and resulting in mould growth and
spoilage.

63. • Solar driers compared to fuel- driers
• The choice between using solar radiation
or fuel-fired dryers using, for instance,
wood, charcoal, diesel, gas or electricity
depends upon the equipment capital cost,
cost of raw material to be dried, operating
costs of running the dryer and the likely
price obtained for the final dried product.

64. • Fuel heating allows much better control
of the drying operation than solar
heating and does not depend on the sun
to be shining.
• However, it is possible to combine solar
drying with a fuel-source to reduce fuel
costs.
• Such systems include pre-heating of air
by solar energy.

65. • Choice of solar dryer
• The choice between alternative types of
solar drier will depend on local
requirements including scale of operation
as well as the budget available.
• If intended for smallholder farmers drying
crops for their own needs then capital cost
may well be the main constraint and so
low-cost plastic-covered tent or box driers
may be the most suitable choice.

66. • However, commercial farmers with an
assured market for their product may
consider banks of fan-assisted, glass-
covered solar dryers more appropriate
for their needs.