Solar Thermal Technologies

UNIT II SOLAR THERMAL TECHNOLOGIES

Working Principle

Principle – To convert solar energy into heat energy by absorbing it.Principle components:

• Solar collector to covert energy efficiently• Medium for energy transport• Water/ air /others• Storage system to overcome the mismatch between energy available and demand• Systems to transport and use energy/ medium • Control systems

Types of Solar systemThus energy from the sun can be categorized in two ways: •In the form of heat (or thermal energy), and •In the form of light energy.

Solar energy can be converted to thermal (or heat) energy and used to: • Heat water – for use in homes, buildings, or swimming pools.• Heat vessels – for cooking food• Heat spaces – inside greenhouses, homes, and other buildings.

Types…

Solar energy can be converted to electricity in two ways:• Photovoltaic (PV devices) or “solar cells” – change sunlight directly into electricity. • Solar Power Plants - indirectly generate electricity when the heat from solar thermal collectors is used to heat a fluid which produces steam that is used to power generator.

Design of solar water heating system

A solar water heating system consists of:•Fat plate solar collector, •Insulated storage tank kept at a height behind the collector, •Insulated hot water carrying pipes and •Cold water carrying pipes. The system is generally installed on the roof or on open ground, with the collector facing the sun and connected to a continuous water supply.

Designs…

The flat plate collector comprises – front glass glazing – blackened metallic (copper) absorber sheet with

built-in channels or riser tubes welded to it to carry water.

– The entire assembly is placed in a flat metallic box.

Procedure for Operation and Maintenance

• Check water in the cold water supply line and also storage tank.

• Check the collector glazing. It should not be dirty. In case dirt is deposited on the glazing it should be cleaned.

• Open all the control valves as suggested by the supplier• Check electrical supply.• Check the water pump. It should not be jammed.• Check whether the Non Return Valve (NRV) on the cold

water inlet side is working.• Ensure the lever of mixer in the bathroom is on either left or

right side AND NOT IN • THE MIDDLE to avoid mixing of hot / cold water

Operation of the Solar Water Heater

• After ascertaining that the requirements indicated above are fulfilled, start the circulating pump.

• The water will start circulating in the collector system and after getting heated will be collected in the storage tank.

• The temperature of water in the storage tank will rise which can be seen in the temperature indicators provided in the system.

• There is a possibility of air locking in collector riser tubes and headers which can be released by increasing the flow of water by opening control valves.

• As soon as the control valves are opened beyond the limits suggested, water will start coming out from the air vents.

• In a THERMOSYPHONIC SYSTEM hot water will start collecting in the storage tank as soon as sunlight falls on the collector and water is available in the system

Classification:

Solar water heating systems are classified in 2 types based on the circulation of water.

•Thermosyphon system.

•Forced flow or forced circulation system

Thermosyphon system:• In the Thermosyphon system, water comes from the over head tank

to bottom of solar collector by natural circulation and water circulates from the collector to storage tank as long as the absorber keeps absorbing heat from the sun and water gets heated in the collector.

• The cold water at the bottom of storage tank runs into the collector and replaces the hot water, which is then forced inside the insulated hot water storage tank.

• The process of the circulation stops when there is no solar radiation on the collector.

• Thermosyphon system is simple and requires less maintenance due to absence of controls and instrumentation.

• Never use a Thermosyphon system for a large set-up.

Thermosyphon Heating system:

Thermosyphon Cooling system

Forced circulation system:

• In the forced flow system, a pump is used for circulating water between the collectors and the insulated hot water storage tank.

• The forced flow systems are more efficient as compared to Thermosyphon systems due to higher flow rate.

• Generally, the pumps are operated by differential temperature control (DTC) system, which senses the pre-setting temperature difference between inlet and outlet of the collectors.

Thermal Energy storage systems

• TES has attracted increasing interest for thermal applications such as space heating, hot water, cooling, and air-conditioning.

• TES systems have the potential for increasing the effective use of thermal energy equipment and for facilitating large-scale fuel switching.

• There are mainly two types of TES systems;– sensible (e.g., water and rock)– Latent (e.g., water/ice and salt hydrates)

• The selection of a TES system mainly depends on the storage period required.– seasonal, economic viability, operating conditions

TES System…

• TES is basically the temporary “holding” of energy for later use.

• Application of TES– solar energy for night and weekend use, – of summer heat for winter space heating, and – Of ice from winter for space cooling in summer,– the heat or cool generated electrically during– off-peak hours can be used during subsequent

peak demand hours

Principle (TES):

• A complete storage process involves at least three steps: charging, storing, and discharging.

Design Considerations for Solar Energy-Based TES

• The energy from solar collectors tends to be at low temperatures and requires a large storage mass when stored as sensible heat.

• a space-conditioning system using solar energy needs to be supplemented by an electric- or fuel-powered auxiliary energy source.

• the quantity of solar energy to be stored.

Solar cooker

Solar cooker is a device which cooks food without any conventional cooking fuel/energy like cow dung, wood, coal, gas, kerosene or electricity. It converts sunlight into heat energy and cooks the food.

Basic principles of solar cooking

• Solar cookers utilise the simple principles of reflection, concentration, glazing, absorption and the greenhouse effect to produce heat.

• Basically a solar cooker is an insulated container with a multiple or single glass (or other transparent material) cover.

• This kind of cooker depends on the “greenhouse effect” in which the transparent glazing permits passage of shorter wavelength solar radiation

• Mirrors may be used to reflect additional solar radiation into the cooking chamber.

• A double-walled insulated box can also serve to hold the heat inside the cooker.

• Box solar cooker

Description:• The Outer Body• The Inner Cooking Box/Tray• Thermal Insulation• The Double Glass Lid• Mirror• Cooking pots• Side window

• Dish solar cooker

Design of dish cooker

• A typical dish solar cooker has a minimum aperture diameter of 1.4 m and a focal length of 0.28 m.

• The parabolic dish, made of single reflector, or by joining smaller pieces of reflector, is fixed firmly to a rigid frame.

• Size and shape of the dish should be such that when exposed to sun in the normal direction, a point focus would be formed.

• The reflecting material used for fabrication of this cooker is anodized aluminium sheet, which has a reflectivity of over 80%.

Material required for fabrication:

• Supporting Frame of the dish

• Stand for the dish

• Tracking Mechanism

• Cooking vessel (Optional)

Community Solar Cooker (for indoor cooking)

• The community solar cooker too is a parabolic reflector cooker.

• However, it is larger than the SK-14 type, and is known as a Scheffler cooker.

• The unique feature of this cooker is that it makes possible to cook using solar energy within the kitchen itself.

Design of community solar cooker• The community solar cooker consists of solar concentrator

with minimum 7.0 & 9.5 sq.m aperture area, which is known as primary reflector.

• A mechanical clock type tracking arrangement is provided to keep the dish facing the Sun.

• A secondary reflector further concentrates the rays on to the bottom of the pot/frying pan (painted black to absorb maximum heat).

• The dish is commonly known as “Scheffler” dish, which has been found to be useful for generating heat to cook food for 50 to 75 persons in community kitchens.

• The mechanical clockwork arrangement rotates the primary reflector outside to automatically track the sun.

Solar Powered Desalination

World Water Resources

Salt Water

Fresh Water The Worlds Water 97% Sea Water 3% Fresh Water

Desalination

The separation and removal of ions, salts and other dissolved solids from water. Heat BasedMembrane Based

Two methods of desalination:Direct Indirect

Process Technology (1)

Process methodology

UNTREATED SOURCE WATERUNTREATED SOURCE WATER

MECHANICAL PRE-TREATMENTMECHANICAL PRE-TREATMENT

SAND FILTERSSAND FILTERS

REVERSE OSMOSISREVERSE OSMOSIS

DESINFECTIONDESINFECTION

REMINERALIZATIONREMINERALIZATION

TREATED DRINKING WATERTREATED DRINKING WATER

Solar pond

• Solar energy systems must incorporate storage in order to take care of energy needs during nights and on cloudy days.

• One way to overcome these problems is to use a large body of water for the collection and storage of solar energy. This concept is called a solar pond.

Solar ponds

Principle of a solar pond

• In a clear natural pond about 30% solar radiation reaches a depth of 2 metres.

• This solar radiation is absorbed at the bottom of the pond. • The hotter water at the bottom becomes lighter and hence

rises to the surface.• Here it loses heat to the ambient air and, hence, a natural

pond does not attain temperatures much above the ambient.• If some mechanism can be devised to prevent the mixing

between the upper and lower layers of a pond, then the temperatures of the lower layers will be higher than of the upper layers.

• The simplest method is to make the lower layer denser than the upper layer by adding salt in the lower layers.

Solar drying

Solar dryers are devices that use solar energy to dry substances, especially food. There are two general types of solar dryers: Direct and indirect

Direct solar dryers expose the substance to be dehydrated to direct sunlight. They have a black absorbing surface which collects the light and converts it to heat; the substance to be dried is placed directly on this surface. These driers may have enclosures, glass covers and/or vents to in order to increase efficiency.

• In indirect solar dryers, the black surface heats incoming air, rather than directly heating the substance to be dried.

• This heated air is then passed over the substance and exits through a chimney, taking moisture from the substance with it.

Classification of solar dryers:

• Passive solar dryers– Tent dryers

Tent dryers….

• Tent dryers are cheap and simple to build and consist of a frame wood poles covered with plastic sheet.

• Black plastic should be used on the wall facing away from the sun.

• The food to be dried is placed on a rack above the ground.

Box dryers• The box type solar dryer has widely used for

small food drying.• It consists of a wooden box with a transparent

lid.• The inside is painted is black and the food

supported on a mesh tray above the dryer food.

Seesaw dryer• The traditional seesaw dryer has a rigid, rectangular frame, the length of

which being 3 times the width resting on a support with an axis.• This support is oriented north-south and is sufficiently high to allow the

frame to be tilted 30 0 . • The material for drying is placed on a number of trays, that allow vertical

air circulation and maximum evaporation.

Cabinet dryer• The crops is located in tray or shelves inside a drying

chamber.• If the chamber is transparent, the dryer is termed an

integral type or direct solar dryer.• If the opaque, the dryer is termed distributed type.• Mixed mode dryers combine the feartures of the

integral type and distributed type.

Active solar cabinet dryers

Active ventilated cabinet solar dryers:– If utility electricity is available it is cheaper to

connect the fans to the grid, compared to a connection to the grid.

– In a PV-powered system, the fan is directly coupled to the solar module, working without an accumulator and load controller.

– Increasing solar radiation increases the modules output, thus speeding up the fan.

Active ventilated cabinet solar dryers

Cabinet dryer with backup heating

• Natural convection dryer with additional biomass back-up heater– one exception is the dryer reported by which

used a sawdust burner to provide heat during poor weather and at night.

Natural convection dryer with additional biomass back-up heater

Greenhouse dryers

• The idea of a greenhouse dryer is to replace the function of the solar collector by a green house system.

• The roof and wall of this solar dryer can be made od transparent materials such as glass, fibre glass, UV stabilized plastic.

Natural convection greenhouse dryer:

The earliest form of practically-realised natural circulation solar greenhouse dryer.

The consisted of two parallel rows of drying platforms of galvanised iron wire mesh surface laid over wooden beams.

Greenhouse dryer with forced ventilation

Tunnel dryers

Three major dryer components;Solar collector, food dryer compartmentAirflow system.

Drying behavior

Apart from weather conditions the drying behavior of agricultural crops during drying depends on the:– Product – Size and shape– Initial moisture content– Final moisture content – Bulk density– Thickness of the layer– Mechanical or chemical pre-treatment– Turning intervals– Temperature of grain