Renewable Energy/Solar Thermal

Introduction

Group of concepts for renewable energy where heat from the solar energy is utilized to generate energy are considered under Solar Thermal section. This is one of the most used concept for the domestic purpose and one can reach out in local market for products which are efficient and cost effective for local installation.

Domestic Application

Domestic applications are such where there can be installation on each house or a building to utilize the Solar Thermal energy. These are mainly for domestic heating purpose. There are products readily available in market for heating and cooking.

Water Heating

Solar radiation can be converted into useful energy directly by using a solar collector to heat water. This can be used to provide hot water for direct use or heating. Solar heat can also be used to for room heating.

Since solar energy is available only for the about 6 hr a day there is always the need for attached energy storage with this renewable energy collectors. With solar water heater there is a thermally insulated tank which collect hot water generated during the daytime for use at appropriate time. Such system can also be electricity/gas boosted.

Construction

This section will explain in simple term what are basic components and type of constructions for water heating using solar thermal.

Cooking

Since cooking is done in day time there is usually no need to have an attached energy storage with this type of installation.

Construction

This section will explain in simple term what are basic components and type of constructions for cooking using solar thermal.

Electricity Generation

Most of the commercial solar thermal implementation are for generation of electricity. The principle used in electricity generation is to have a fluid heated by solar heat. Then use the heated fluid in a turbine to generate electricity. To run a turbine the fluid needs to be heated to very high temperature and high pressure needs to be achieved. Achieving such high temperature requires a much more complicated collector.

Direct Solar

This section covers such concepts where only solar thermal energy is used to generate electricity.

Hybrid Solar

This section covers such concepts where solar thermal energy is used along with other conventional power generation plant to generate electricity. This is more convenient option for the commercial generation of electricity as the need of electricity is different in different time of day. Also hybrid installation can complement and increase the efficiency of the conventional power plant.

Experimental

This section gives details of the solar thermal concepts which are more under the research or experimental stage.

Solar Pond

A solar pond is the pond of gradient salinity which traps the heat from sun.Any lake absorbs heat from the sun. Normally, heat is lost as warm water rises to the surface and cools by evaporation. But water is a VERY poor conductor of heat and if this circulation can be stopped, the heat can be trapped in the bottom of the lake. A salt lake, (ideally about 3m deep), managed so that the water on top is of (relatively) low salinity and the water on the bottom is of very high salinity, will not circulate to release heat because the water on the bottom is so heavy with salt it cannot rise. The deeper water gets very hot - to over 100 degrees in the right circumstances - 80 degrees is common in the tropics. In Southern Australia 60 degrees C is easily achievable - even in winter. The main management problem is to extract heat at the right rate so the lake does not boil or 'turn over' and lose its heat. By utilizing the sun energy we make useful this storage of hot water in our various task in which the power generation is main. As a form of solar energy, the solar pond collector has major advantages.

   The heat storage is massive, so energy can be extracted day and night - hence it is a source of 'base load' solar power  -
   no batteries or other storage needed !

   Solar ponds can have very large heat collection area at low cost.

   The major production potential is during peak electrical power demand (and price) in mid summer

   The technology and scientific principles for collection and extraction of heat and its conversion to electricity 
   are well understood and well documented in scientific papers.

   Any qualified engineer would be able to build one of these systems (being a refrigeration specialist would be 
   useful if you wanted to build a Rankine engine)

Types of Solar Ponds

There are two main categories of solar ponds: non-convecting ponds, which reduce heat loss by preventing convection from occurring within the pond; and convecting ponds, which reduce heat loss by hindering evaporation with a cover over the surface of the pond.

Non-convecting Ponds

There are two main types of non-convecting ponds: salt gradient ponds and membrane ponds. A salt gradient pond has three distinct layers of brine (a mixture of salt and water) of varying concentrations. Because the density of the brine increases with salt concentration, the most concentrated layer forms at the bottom. The least concentrated layer is at the surface. The salts commonly used are sodium chloride and magnesium chloride. A dark-colored material—usually butyl rubber—lines the pond. The dark lining enhances absorption of the sun’s radiation and prevents the salt from contaminating the surrounding soil and groundwater. As sunlight enters the pond, the water and the lining absorb the solar radiation. As a result, the water near the bottom of the pond becomes warm—up to 200o F (93.3oC). Although all of the layers store some heat, the bottom layer stores the most. Even when it becomes warm, the bottom layer remains denser than the upper layers, thus inhibiting convection. Pumping the brine through an external heat exchanger or an evaporator removes the heat from this bottom layer. Another method of heat removal is to extract heat with a heat transfer fluid as it is pumped through a heat exchanger placed on the bottom of the pond. Another type of non-convecting pond, the membrane pond, inhibits convection by physically separating the layers with thin transparent membranes. As with salt gradient ponds, heat is removed from the bottom layer.

Convecting Pond

A well-researched example of a convecting pond is the shallow solar pond. This pond consists of pure water enclosed in a large bag that allows convection but hinders evaporation. The bag has a blackened bottom, has foam insulation below, and two types of glazing (sheets of plastic or glass) on top. The sun heats the water in the bag during the day. At night the hot water is pumped into a large heat storage tank to minimize heat loss. Excessive heat loss when pumping the hot water to the storage tank has limited the development of shallow solar ponds. Another type of convecting pond is the deep, salt-less pond. This convecting pond differs from shallow solar ponds only in that the water need not be pumped in and out of storage. Double-glazing covers deep salt-less ponds. At night, or when solar energy is not available, placing insulation on top of the glazing reduces heat loss.

Solar ponds can only be economically constructed if there is an abundance of inexpensive salt, flat land, and easy access to water. Environmental factors are also important. An example is preventing soil contamination from the brine in a solar pond. For these reasons, and because of the current availability of cheap fossil fuels, solar pond development has been limited in the United States. The greatest potential market for solar ponds in this country could be in the industrial process heat sectors