Question

Describe the structure and principle of operation of flat plate and evacuated tube solar collectors (both water heating and air heating) Determine key performance parameters for a flat plate collector from collector test results C Describe the structure and principle of operation of the key storage elements of low temperature solar thermal systems -namely hot water storage tanks and rock beds C o Define and use the various angles used to specify solar position and the angle of incidence of direct sunlight on an arbitrary surface Describe the characteristics of solar radiation at the earth's surface in terms of magnitude, clearness index and spectral characteristics o Describe how solar radiation is measured and the advantages and disadvantages of the different types of pyranometers o Set up a pyranometer to measure solar radiation o Describe the process by which, given the solar radiation on a horizontal surface, the solar radiation on a tilted surface can be calculated, o Use a spreadsheet to calculate the monthly average daily solar radiation on a tilted surface given the monthly average daily solar radiation on the horizontal surface Explain the concept of utilisability and why it can be a useful solar radiation statistic o Characterise the solar radiation resource for a particular location, given certain key data O Describe the structure and principle of operation of close-coupled thermosyphon solar hot water systems Describe the components of, and the operation of, an active solar hot water heating system Use software to predict the performance of a thermosyphon solar hot water system Use software such as f-Chart and RETScreen to estimate the performance of solar thermal systems Describe the issues in the design of large solar thermal systems relating to pipework design, insulation of piping and tanks, pump selection and control of active systems

Answer 1

Flat plate collectors

Flat-plate collectors are the most common solar thermal technology in Europe.They consist of an (1) enclosure containing (2) a dark colored absorber plate with fluid circulation passageways, and a transparent cover to allow transmission of solar energy into the enclosure. The sides and back of the enclosure are typically insulated to reduce heat loss to the ambient. A heat transfer fluid is circulated through the absorber's fluid passageways to remove heat from the solar collector. The circulation fluid in tropical and sub-tropical climates is typically water. In climates where freezing is likely, a heat transfer fluid similar to an automotive antifreeze solution may be used instead of water, or in a mixture with water. If a heat transfer fluid is used, a heat exchanger is typically employed to transfer heat from the solar collector fluid to a hot water storage tank. The most common absorber design consists of copper tubing joined to a high conductivity metal sheet (copper or aluminum). A dark coating is applied to the sun-facing side of the absorber assembly to increase it absorption of solar energy. A common absorber coating is black enamel paint.

In higher performance solar collector designs, the transparent cover is tempered soda-lime glass having reduced iron oxide content same as for photovoltaic solar panels. The glass may also have a stippling pattern and one or two anti-reflective coating to further enhance transparency. The absorber coating is typically a selective coating, where selective stands for having the special optical property to combine high absorption in the visible part of the electromagnetic spectrum coupled to low emittance in the infrared one. This creates a selective surface, which reduces black body energy emission from the absorber and improves performance. Piping can be laser or ultrasound welded to the absorber sheet to reduce damage to the selective coating, which is typically applied prior to joining to large coils in a roll-to-roll process.

Absorber piping configurations include:

harp: traditional design with bottom pipe risers and top collection pipe, used in low pressure thermosyphon and pumped systems;
serpentine: one continuous s-shaped pipe that maximises temperature but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role);
flooded: consisting of two sheets of metal molded to produce a wide circulation zone that improves heat transfer;
boundary layer: consisting of several layers of transparent and opaque sheets that enable absorption in a boundary layer. Because the energy is absorbed in the boundary layer, heat conversion may be more efficient than for collectors where absorbed heat is conducted through a material before being accumulated in the circulating liquid.[citation needed]
A flat plate collector making use of a honeycomb structure to reduce heat loss also at the glass side too has also been made available commercially.[7] Most flat plate collectors have a life expectancy of over 25 years.

Low-temperature solar thermal installations are considered those installations of solar thermal energy that provide useful heat at temperatures below 65 ° C through solar energy.

A low-temperature solar thermal installation consists of solar collectors, two water circuits (primary and secondary), heat exchanger, accumulator expansion vessel and pipes.

The circulation of water inside the circuits can be obtained by thermosiphon, taking advantage of the density difference of the water at different temperatures or by means of a circulation pump. Although with a circulation pump an external contribution of electrical energy is needed.

Applications of solar thermal energy
Low-temperature solar thermal energy is a form of renewable energy that can be applied in many areas in order to reduce dependence on other sources of non-renewable energy such as natural gas or coal combustion.

Solar thermal systems at low temperature are useful in applications such as heating swimming pools, for domestic use (hot water and heating), for industrial uses that require hot water but not at a higher temperature of 65ºC.

Solar hot water heaters can reduce your household carbon emissions by more than 20%, besides cutting your energy bills drastically. Choosing a style of solar hot water heater can be quite tricky, and it's a great idea to get a solar professional to work through your options. However, here's an explanation about close-coupled thermosiphon (mains pressure) systems.

This is the type that you'll see most commonly - you'll see roof mounted collectors with a horizontal tank about the panels. Usually, no pump is needed because heated water naturally rises through the collectors and into the storage tank. When the heated water enters the tank, cold water in the tank is forced into the collectors and is then heated. This cycle happens over and over throughout the day while the sun shines.

storage tank booster (gas ar electric) hot water from collector to tank hot water to house trans parent mains inlet COver dump valve collector cases absorber plate Figure 1: Close-coupled thermosyphon (mains pressure)

Solar heating systems are classified as “active” or “passive” solar heating systems, or a combination of both. We will first look at active systems.

Active solar heating systems are comprised of collectors, a distribution system, and a storage device.

Solar Collectors Sensor Storage Tank with heat exchanger Hot Water Outlet Controller Pressure Gauge Pump Cold Water Inlet

Active solar heating systems operate as follows:

Flat plate collectors are usually placed on the roof or ground in the sunlight. The top or sunny side has a glass or plastic cover to let the solar energy in. The inside space is a black (absorbing) material to maximize the absorption of the solar energy.
Cold water is drawn from the storage tank by pump #1 and is pumped through the flat plate collector mounted on the roof of the house.
The water absorbs the solar energy and is returned back to the tank.
Warm water from the tank is pumped by pump #2 though the heating coil.
The fan blows air (from the room) over the heated coil, and the heated air then passes into the room and heats the room.
Cold air sinks to the bottom and is recirculated over the heating coil.