::SOLAR THERMAL TECHNOLOGIES::

&

APPLICATIONS

 

In solar thermal technologies solar energy can be utilized directly by exploiting its heat characteristics, such technologies are comparatively simple, relatively low cost and easy to adopt. The applications include off and on grid power generation, cooking, heating and cooling of buildings, generation of high temperature steam, heating water for domestic and industrial applications, and drying agricultural products under controlled temperatures. Solar Thermal power uses direct sunlight, called ‘infrared radiation’ or Direct Normal Irradiation (DNI). This is the sunlight that is not deviated by clouds, fumes or dust in the atmosphere and which reaches the Earth’s surface in parallel beams for concentration. The following are the application of solar thermal technologies:

 

Solar Dryer

* Solar Drying

* Drying of Fruits & Vegetables

 

Solar dryers require a certain investment for the set-up of the appliance, but no expenditures for the fuel. The basic function of a solar dryer is to heat air to a constant temperature with solar energy, which facilitates extraction of humidity from crops inside a drying chamber. Ventilation is enabled at a constant rate through defined air inlets and outlets, small solar ventilators or temperature difference, either due to exposition or vertical height. In direct sun driers the food is put in boxes with a transparent lid. Additionally, the temperature in the drier is raised due to the greenhouse effect and the air exchange is regulated by vents. The food is not exposed to direct sunlight in indirect sun driers as the fresh air is heated separately from the food chamber. This method is preferable for drying foods which lose nutritional value when exposed to direct sunlight. Hybrid driers combine solar energy with a fossil fuel or biomass fuel.

 

A first step when considering solar drying is to compare the different drying options available. Solar drying will only be successful, when it shows tangible benefits in comparison to existing drying methods. In comparison to the traditional way of drying outside in an open field, solar dryers prevent contamination of produce by dust, insects, etc., thereby ensuring quality. They allow small-scale farmers to transform their harvest into storable and tradable goods, which they can sell off-season at higher prices. The constant temperature and ventilation allows a consistent drying process which results in better product quality and higher prices. However, the investments costs of solar dryers vary highly depending on the size of the solar dryer, locally available materials and environmental conditions, such as slope and exposition of the side, rainy seasons.

There are different types of solar dryers, such as direct drying (solar box dryer), indirect drying (solar cabinet dryer), mixed mode drying (solar tunnel dryer) or hybrid drying (hybrid solar/biomass cabinet dryer). Small-scale solar box and cabinet dryers are based on natural air convection, while solar tunnel dryer is based on forced convection (air circulation fan necessary).

 

In Pakistan due to the lack of logistics and basic infrastructure in the northern mountainous regions of Gilgit and Sakardu etc tons of fruit like apricots go waste every year. Solar dryers are now being used to dry large quantities of such fruit, which are transported and sold them later in the urban market, resulting in a positive effect on the economy of this area. Solar dryers could be equally effectively used in the provinces of Punjab and Sindh to dry agriculture products for better market value and generating local employment.

 

Achievement of PCRET in solar drying, several models of solar dryers have been developed by this council and are being disseminated.

 

Ř  In Public Sector Development Programme, Govt of Pakistan PCRET designed developed and installed 10 unglazed collector type hybrid (Solar + Biomass) dryer having 500 kg capacities for drying of dates in Punjab, Khyber Pakhtoonkhwa.

Ř  Designed solar dryer for Pakistan Tobacco Board for Curing of Tobacco.

Ř  Designed 500 kg capacity solar tunnel dryer for drying of dates for Govt of Sindh.

Ř  Designed and developed Green House Solar Dryer of 200-300 kg capacity.

 

Solar Water Heater

 

Solar water heating or SWH is the process of converting sunlight into energy that can then be used for domestic water heating. This heated water can be used for washing in the home, radiant floor heating, or to heat swimming pools.

Solar water heating systems use a large number of different technologies and these technologies can be used almost anywhere in the world. Each of these systems include two main components - storage tanks and solar collectors. Additionally, there are two main types of solar water heating systems: active systems with circulating pumps and passive systems without pumps.

Types of Systems

Generally speaking, solar water heating systems are composed of two main pieces: storage tanks and solar collectors. The collectors serve the purpose of collecting and retaining heat from the Sun. Once the collectors capture the energy from the Sun, this heat is transferred to a liquid known as the heat transfer fluid. If this fluid is water it takes the heat from the collector and moves the warm water for use or storage. However if this fluid is not water, heat exchangers are used to transfer the heat from the heat exchange fluid to a home's water supply. Pumps are sometimes used in active heating systems to control what temperature the water gets to and how quickly it moves.

Passive

Passive solar water heating, in general, is less expensive than active solar water heating as it does not require a pump. Additionally, this makes maintenance of the system easier. However, they are usually less efficient although they may last longer. The two main types of passive water heating systems are:

§  Integral Collector-Storage Systems: These types of systems work better in areas that rarely fall below freezing, or in households with significant daytime or evening hot water needs. These systems include one or more black tanks in an insulated box. Cold water passes through the solar collector, heating the water. The slightly warm water then moves to a backup water heater, providing a reliable source of hot water. These are used in areas that do not freeze as sub-zero temperatures could result in the pipes freezing.

§  Thermosyphon Systems: Fluid movement in this system occurs when warm water rises and cool water sinks. When the water is heated, hot water flows into the storage tank. These are usually more expensive as roof design must be taken into account because of the heavy storage tank.

Active

There are two distinct types of active solar water heating systems that utilize pumps. They are:

§  Direct circulation systems: In these systems, pumps circulate water through collectors and into the home. These systems work well in climates where it doesn't reach freezing temperatures or else the water could freeze and break the system.

§  Indirect circulation systems: These systems pump a heat-transfer fluid with a very low freezing point (such as glycol) through collectors and into a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures as the fluid will not freeze and break the system.

Benefits

Using the energy from the Sun to heat water can save homeowners significant amounts of money in the long run. If solar is used to replace all or part of the water heating in a home, there can be significant savings on the homes energy bill. This depends largely on how much the Sun shines where the collector is located, and the price of energy. As well, the use of these solar water heaters can result in provincial, state, or federal tax credits or utility rebates.

The fact that energy used is reduced with the use of solar water heaters is also beneficial to the environment. Currently, most energy people use comes from burning a variety of fossil fuels. This combustion of fossil fuel resources is associated with a wide array of environmental impacts including climate change, acid rain, and global warming associated with carbon dioxide emissions. The use of solar water heating reduces energy use from these fossil fuel sources, benefiting the environment in the long run.

Solar Desalination

 

Solar-powered desalination unit, device that transforms salt water into drinking water by converting the Sun’s energy to heat, directly or indirectly, to drive the desalination process. Solar desalination mimics Earth’s natural water cycle (the process that generates rainfall) and has been practiced by humans as a rudimentary water-treatment process since the time of the ancient Greeks.

 

The design of a direct, or passive, unit, generically called a solar still, can be quite simple and inexpensive. The salt water in the desalination unit is heated by the Sun, converting the liquid to water vapour (a gas). As it is heated, the water vapour rises to the top of the unit, collects on the inside lid, and condenses back to liquid as fresh water in a separate collection container. The salt cannot change to a gas and, therefore, remains in the original unit. Direct solar desalination works well for purification but, because of the low operating temperature of the unit, does not produce a lot of water per day. The amount of drinking water produced in a direct desalination unit is proportional to the surface area of the device. The daily freshwater output per square meter of area is typically 2 to 3 liters (about 0.5 to 0.8 gallon), depending on the solar still design. The typically easy-to-operate design, however, makes it ideal for small-scale needs of families in remote areas, since the average person needs about two liters of water per day to survive. The process is driven solely by solar energy, so weather conditions and variable solar intensity (due to the shifting position of the Sun throughout the day) can negatively impact efficiency.

The output from a direct solar desalination unit is too low to be employed in a commercial operation. Therefore, indirect solar desalination methods must be utilized to increase freshwater production. Indirect solar desalination combines two different technologies: solar energy collection (through the use of photovoltaic panels) is coupled with a proven desalination method such as multistage flash (MSF) distillation, multiple effect evaporation (MEE), or reverse osmosis (RO). Employing renewable solar energy as a supplemental heat source can help to eliminate energy consumption of fossil fuels, significantly reducing operating costs and making commercial desalination plants viable.

 

A small fraction of the fresh water produced globally through desalination uses solar energy. As the demand for fresh water increases and advancements in solar technology (such as high-concentrated photovoltaic and thermal energy storage systems) become commercially feasible, solar desalination plants are likely to become more widespread.

 

In Pakistan, the unavailability of drinkable water in large parts of Baluchistan, Sindh and Southern Punjab is a critical issue. Underground water exists, but it is highly saline due to the presence mainly of sodium chloride and other salts. Saline water is not only unsuitable for washing and cleaning but also causes many stomach diseases and hypertension.

Solar Cooker

 

A solar cooker is a device that uses sunlight from energy. They use no fuel and cost nothing to run. They also help slow deforestation and desertification. Another benefit of solar cooking is that unlike cooking with fire, solar cookers don't pollute the air. Solar cookers are also sometimes used for cooking outside, especially when using fire is risky or there is no fuel. The solar cooker is very useful. It is a renewable source of energy. Solar cookers come in two types: box types that are more suitable for baking the food, and concentrated (parabolic) type that works at high temperatures. Foods prepared in solar ovens keep their natural flavor and taste and retain their vitamin content. Less oil is required for cooking in the solar oven, therefore, solar oven cooked food is good for weight conscious people.

 

All Pakistani dishes including prathas and chapatti can be prepared on concentrated type cooker in the same time interval as taken by a conventional single stove gas cooker. The method of cooking is also similar to that of conventional cooker. The cooker emits no harmful smoke to harm eyes and lungs of the cook. The cooking can be done on this cooker from 9.00 am to 3.00 p.m. on all sunny days of the year.

 

A number of public sector organizations have worked in the past and are still working on the development of low cost and efficient designs of both box and concentrator type solar cookers. Non-governmental organizations are also active in this field and have supplied a number of such cookers to rural areas. The numbers of solar cookers in use in Pakistan is more than 10,000, but it is still far less than that being used in China and India (400,000). Pakistan needs to popularize solar cookers to reduce the use of precious forest resources as fuel wood.

 

Solar Thermal Power Plants

 

Solar thermal power plants are electricity generation plants that utilize energy from the Sun to heat a fluid to a high temperature. This fluid then transfers its heat to water, which then becomes superheated steam. This steam is then used to turn turbines in a power plant, and this mechanical energy is converted into electricity by a generator. This type of generation is essentially the same as electricity generation that uses fossil fuels, but instead heats steam using sunlight instead of combustion of fossil fuels. These systems use solar collectors to concentrate the Sun's rays on one point to achieve appropriately high temperatures.

There are two types of systems to collect solar radiation and store it: passive systems and active systems. Solar thermal power plants are considered active systems. These plants are designed to operate using only solar energy, but most plants can use fossil fuel combustion to supplement output when needed.

Types of plants

Despite the fact that there are several different types of solar thermal power plants, they are all the same in that they utilize mirrors to reflect and concentrate sunlight on a point. At this point the solar energy is collected and converted to heat energy, which creates steam and runs a generator. This creates electricity.

 

Parabolic Troughs

These troughs, also known as line focus collectors, are composed of a long, parabolic shaped reflector that concentrates incident sunlight on a pipe that runs down the trough. The collectors sometimes utilize a single-axis Solar tracking system to track the Sun across the sky as it moves from east to west to ensure that there is always maximum solar energy incident on the mirrors. The receiver pipe in the center can reach temperatures upward of 400°C as the trough focuses Sun at 30-100 times its normal intensity.

These troughs are lined up in rows on a solar field. A heat transfer fluid is heated as it is run through the pipes in the parabolic trough. This fluid then returns to heat exchangers at a central location where the heat is transferred to water, generating high-pressure superheated steam. This steam then moves a turbine to power a generator and produce electricity. The heat transfer fluid is then cooled and run back through the solar field.

 

Parabolic Dishes

 

These are large parabolic dishes that use motors to track the Sun. This ensures that they always receive the highest possible amount of incoming solar radiation that they then concentrate at the focal point of the dish. These dishes can concentrate sunlight much better than parabolic troughs and the fluid run through them can reach temperatures upwards of 750°C.

In these systems, a Stirling engine coverts heat to mechanical energy by compressing working fluid when cold and allowing the heated fluid to expand outward in a piston or move through a turbine. A generator then converts this mechanical energy to electricity.

 

Solar Towers

 

Solar power towers are large towers that act as a central receiver for solar energy. They stand in the middle of a large array of mirrors that all concentrate sunlight on a point in the tower. These large numbers of flat, sun tracking mirrors are known as heliostats. In the tower, there is a mounted heat exchanger where the heat exchange fluid is warmed. The heat concentrated to this point can be 1500 times as intense as incident sunlight. The hot fluid is then used to create steam to run a turbine and generator, producing electricity. One drawback with these towers is they must be very large to be economical.

 

Benefits and Drawbacks

Because these systems can generate steam of such high temperatures, the conversion of heat energy to electricity is more efficient. As well, these plants get around the issue of being unable to efficiently store electricity by being able to store heat instead. The storage of heat is more efficient and cost-effective than storing electricity.

Additionally, these plants can produce dispatch able base load energy, which is important as it means these plants produce a reliable amount of energy and can be turned on or up at will, meeting the energy demands of society. In addition to this, solar thermal power plants represent a type of electricity generation technology that is cleaner than generating electricity by using fossil fuels. Thus, these are some of the cleanest options for generating electricity. Despite this, there are still associated environmental effects of these plants as a full life cycle analysis can show all associated carbon dioxide emissions involved in the building of these plants. However, emissions are still much lower than those associated with fossil fuel plants.

Some of the drawbacks include the large amount of land necessary for these plants to operate efficiently. As well, the water demand of these plants can also be seen as an issue, as the production of enough steam requires large volumes of water. A final potential impact of the use of large focusing mirrors is the harmful effect these plants have on birds. Birds that fly in the way of the focused rays of Sun can be incinerated. Some reports of bird deaths at power plants such as these amounts the deaths to about one bird every two minutes.