This article mainly focuses on the basics of solar thermal collector. The types of solar thermal collector, maintenance losses and applications of it
- 1 Solar thermal collectors :
- 2 Flat plate solar thermal collector :
Definition of solar thermal collector power system
A solar thermal collector gathers the heat from the solar radiation and gives it to a fluid named as heat transport fluid. The heat is received from the collector by the fluid and delivers it to the thermal storage tank, boiler steam generator, heat exchanger etc. For few hours, the heat is stored by thermal storage system. The heat is released during cloudy hours and at night. Thermal-electric conversion system receives thermal energy and drives steam turbine generator or gas turbine generator. The electrical power is delivered to the electrical load or to the substation. There are many solar thermal energy system applications ranging from simple solar cooker of 1 kW rating to complex solar central receiver thermal power plant of 200 MW rating.
Solar thermal collectors :
A solar thermal collector collects heat by absorbing sunlight. A collector is a device for capturing solar radiation. Solar radiation is energy in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths.
As solar power has low density (kW/m2), therefore large area on the ground is covered by collectors. Flat plate collectors are used for low temperature applications. For achieving higher temperature of transport fluid, the sun rays must be concentrated and focused.
Concentration Ratio (CR):
For flat plate collectors, CR = 1. Using heliostats with sun-tracking in 2 planes, a high CR is obtained which can be of the order of 1000. CR up to 100 can be achieved by using parabolic trough collectors with sun tracking in one plane.
Collector efficiency (n):
The performance of a collector is evaluated in terms of its collector efficiency which is given as
For constant solar radiation (kW/m2), the collector efficiency decreases with the increasing difference between the collector temperature and the outside temperature.
Flat plate solar thermal collector :
One of the widely used types of solar thermal collector is flat plat collector. Flat plate collector absorbs both beam and diffuse components of radiant energy. The absorber plate is a specially treated blackened metal surface. Sun rays striking the absorber plate are absorbed causing rise of temperature of transport fluid. Thermal insulation behind the absorber plate and transparent cover sheets (glass or plastic) prevent loss of heat to surroundings.
Applications of flat plate solar thermal collector:
- Solar water heating systems for residence, hotels, industry.
- Desalination plant for obtaining drinking water from sea water.
- Solar cookers for domestic cooking.
- Drying applications.
- Residence heating.
Losses in flat plate solar thermal collector :
Shadows of some of the neighbor panel fall on the surface of the collector where the angle of elevation of the sun is less than 150 (sun-rise and sunset)
Shadow factor is less than 0.1 during morning and evening. The effective hours of solar collectors are between 9AM and 5PM.
Cosine loss factor
For maximum power collection, the surface of collector should receive the sun rays perpendicularly. If the angle between the perpendicular to the collector surface and the direction of sun rays is θ, then the area of solar beam intercepted by the collector surface is proportional to cos θ.
Reflective loss factor:
The collector glass surface and the reflector surface collect dust, dirt, moisture etc. With the passage of time, the reflector surface is corroded, deformed and its shine fade away. Hence, the efficiency of the collector is reduced significantly with passage of time rust
Maintenance of flat plate solar thermal collector:
- Regular clean up.
- Frequent maintenance
- Yearly overhaul (change of seals, cleaning after dismantling)
Types of flat plat solar thermal collector
Parabolic trough with line focusing reflecting surface provides concentration ratios from 30 to 50 as a result very high temperatures such as 3000 C can be attained.
Light is spotted on the middle of the parabolic trough surface. Heat is absorbed by the pipe which is present alongside of the pipe the working fluid is circulated through the pipe.
Paraboloid dish collectors:
The beam radiation is reflected by paraboloid dish surface. The point focus is obtained with CR (above 1000) and temperatures around 1000OC.