Solar Energy Systems: Collectors, Power Plants, and PV

Posted on Feb 6, 2025 in Geology

Stationary Collectors

Flat-Plate Collectors (FPC)

Consist of the following components:

  1. Glazing: Diathermanous material.
  2. Tubes: To conduct the heat transfer fluid from the inlet to the outlet.
  3. Absorber plates: Flat plate to which the tubes are attached.
  4. Headers: To admit and discharge the fluid.
  5. Insulation: To minimize heat loss.
  6. Container: To surround the other components.

When solar radiation passes through the transparent cover, a portion of this energy is absorbed by the plate and transferred to the transport medium in the fluid tubes to be carried away for use. The underside of the absorber plate is well insulated to reduce conduction losses. The transparent cover is used to reduce convection and radiation losses. The collectors should be oriented directly towards the equator.

Evacuated Tube Collectors (ETC)

Consist of a heat pipe inside a vacuum-sealed tube. It has good performance at high temperatures. The vacuum reduces convection and conduction losses. Use liquid-vapor phase change materials to transfer heat at high efficiency.

Compound Parabolic Collectors (CPC)

The absorber can be cylindrical or flat. Have the capability of reflecting to the absorber all of the incident radiation within wide limits. Are usually covered with glass to avoid dust, this reduces the reflectivity of its walls.

Sun-Tracking Concentrating Collectors

Single Axis

Parabolic Trough Collectors (PTC)

Are parabolic. A metal black tube, covered with a glass tube to reduce heat losses, is placed along the focal line of the receiver. When pointed towards the sun, parallel rays incident on the reflector are reflected onto the receiver tube. PTCs are the most mature solar technology to generate heat at temperatures up to 400ºC or for solar thermal electricity generation.

Linear Fresnel Reflector (LFR)

Array of linear mirror strips which concentrate light onto a fixed receiver mounted on a linear tower. One difficulty is the avoidance of shading and blocking between adjacent reflectors, so the space between them has to be increased.

Two-Axis

Parabolic Dish Reflector (PDR)

Is a point-focus collector that tracks the sun in two axes, concentrating solar energy onto a receiver located at the focal point of the dish. Can reach temperatures above 1500ºC. They are the most efficient of all collector systems.

Heliostat Field Collector (HFC)

A multiplicity of flat mirrors that reflect their incident solar radiation onto a common target. Produce steam at high temperature and pressure. They are highly efficient both in collecting energy and in converting it to electricity. They are quite large.

Passive Solar Energy

Convert solar radiation into heat by means of the building structure itself. Use the building envelope as absorber and the building structure as heat store. System components:

  1. Transparent covers: Transmit a maximum share of solar radiation to the interior and ensure at the same time utmost insulation from the outside.
  2. Absorber and heat storage: They are integrated into the building structure. The room envelopes, with solar radiation exposure, serve as absorber surfaces.
  3. Shading devices: They should be oriented to the south to provide good shade in summer and high irradiation in winter.

Dish/Stirling

It consists mainly of the parabolic-shaped concentrator, a solar receiver, and a Stirling motor. The solar radiation is reflected directly in a receiver located in the focus of the concentrator. The energy of the radiation is transferred to Stirling to convert it into electrical energy.

F-Chart

It is an authorized program for analysis and design of solar systems written by S.A. Klein and W.A. Beckman, the creators. This method provides a means to easily determine the thermal performance of the active solar heating system and the solar hot water system. It is essentially a correlation of the results of hundreds of simulations of the solar heating system.

Solar Thermal Power Plants

The process of solar thermal power generation is:

  1. Concentrating solar radiation by means of a collector system.
  2. Increasing radiation flux density, if applicable.
  3. Absorption of the solar radiation.
  4. Transfer of thermal energy to an energy conversion unit.
  5. Conversion of thermal energy into mechanical energy using a thermal engine.
  6. Conversion of mechanical energy into electrical energy using a generator.

Pmax

The power is calculated from P = I * V. Therefore, at Isc and Voc, the power will be zero, and the maximum value for power will occur between these two values.

FF

Fill Factor (FF) is the ratio of the maximum power to the theoretical power (1). The theoretical power is given by the product of open-circuit voltage and short-circuit current. Typical fill factors range from 0.5 to 0.82.

Effect of Temperature

The efficiency of PV cells depends on the temperature at which they are operating. When a PV cell is exposed to higher temperatures, Isc increases slightly, Voc decreases more significantly, and the effect is the reduction in efficiency. Higher temperatures decrease the maximum power Pmax.

Evolution of PV in Spain

Spain is the fifth country in Europe in terms of cumulative photovoltaic capacity, with 5.4 MW. The largest part was installed in 2008 when the country was the largest market, with more than 3.3 GW. In January 2012, the Spanish government approved Royal Decree 1/12, which suspended the pre-assignment procedures and the introduction of new taxes on electricity generation. In 2013, the photovoltaic capacity was increased by 118 MW.