Solar Energy

There are many different types of systems that will convert the solar resource into a useful form of energy. The first method is to capture the solar energy and convert it into heat which is then supplied to (thermal load) such as house heating, hot water heating or heat for industrial processes.  The second is to collect solar energy as heat and convert it into electricity using a typical power plant or engine.  The third method is to use photovoltaic cells to convert solar energy directly into electricity. The amount  of available energy from the sun varies from location to location and through the year.  NASA's isolation data recorded over a 10-year period provides a good estimate for the amount of solar energy in major locations through the world. 

Low Temperature Solar Thermal

 

Flat-plate thermal solar collectors are the most commonly used type of solar collector. They are easily constructed and operated. A large plate of black material is oriented towards the sun in such a way that the rays that fall on the plate are absorbed and converted to thermal energy. Tubes or pipes are used to remove heat from the plate, transferring it to a liquid (typically water), and carrying it away to the load.  Usually, a transparent (glass or plastic) plates are placed in front of the absorber plate to reduce heat loss to the atmosphere.  Operating temperatures up to 125oC are typical.

Concentrated Solar Thermal Power

When higher temperatures are required, concentrating solar collectors are used. Solar energy falling on a large reflective surface is concentrated onto a smaller area before it is converted into heat. The sun concentration allows the use of a smaller absorbing surface and therefore can reach higher temperatures before heat loss due to radiation and convection wastes the energy that has been collected. Four types of solar concentrators are common: parabolic troughs, parabolic dishes, central receivers and Linear Fresnel Reflector (LFR).  Stalix has been working extensively on reducing the cost of CSP systems.

A parabolic trough concentrates incoming solar radiation onto a line running the length of the trough. A tube (receiver) carrying heat transfer fluid is placed along this line, absorbing concentrated solar radiation and heating the fluid inside. The trough tracks the sun about one axis (usually East-West or North-South). Because the surface area of the receiver tube is small compared to the trough capture area, temperatures up to 400oC can be reached without major heat loss.

 

A parabolic dish concentrates the incoming solar radiation to a point. An insulated cavity containing tubes or other heat transfer devices is placed at the focus point absorbing the concentrated radiation and transferring it to a gas that can reach 1000oC. Parabolic dishes track the sun about two axes.  Dish concentrators typically use Stirling engines which are attached to the receiver at the focus.

 

A central tower system consists of a large field of independently movable flat mirrors (heliostats) and a receiver located at the top of a tower.  Each of the heliostat moves about two axes, throughout the day, to track the sun. The receiver, typically a vertical bundle of tubes, is heated by the reflected sun rays, thereby heating the heat transfer fluid passing through the tubes to temperatures that can reach 1500oC.

A Linear Fresnel Reflector uses a series of long, narrow, shallow-curvature (or even flat) mirrors to focus light onto one or more linear receivers positioned above the mirrors.  The purpose of thiis approach is to offer lower overall costs by sharing a receiver between several mirrors, while still using the simple line-focus geometry with one axis of tracking. When suitable aiming strategies are used, mirrors aim at different receivers at different times of day.  This can allow a denser packing of mirrors on available land area.

 

Photovoltaics

Photovoltaic energy is the conversion of sunlight into electricity through a photovoltaic (PVs) cell, commonly called a solar cell. The PV cells are typically made from Silicon alloys.  Photovoltaic collectors contain an array of individual photovoltaic cells, connected in a series/parallel circuit, and encapsulated within a sandwich structure with the front surface being glass. Solar energy falls directly upon the photovoltaic cell front surface and produces a small direct current voltage, providing electrical energy to a load.

Concentrating Photovoltaics (CPV)
CPV uses lenses and reflectors to concentrate sunlight onto photovoltaic cells, allowing for a decrease in cell size. The main idea is to use very little of the expensive semi-conducting PV material while collecting as much sunlight as possible.

One of the main obstacles with CPV is that the solar cells became very inefficient when exposed to concentrated sunlight. Leveraging its extensive semiconductor and thermal packaging technology, Stalix has come up with solutions that successfully focuses the sun and still maintains highly efficient solar cell output.