Technology Outline

1) Concentrating Solar Power
2) Solar Space Heating
3) Solar Cooling
4) Solar Lighting

1) Concentrating Solar Power

Concentrating solar power (CSP) is a large-scale solar thermal-electric technology that makes use of different kinds of mirror configuration to convert the sun's energy into high-temperature heat. The heat energy is then used to generate electricity in a steam-turbine-driven electricity generator.

Major types of CSP technologies

There are three major types of CSP technologies: (a) Trough System; (b) Dish/Engine System; and (c) Power Tower.

(a) Trough System

This system uses parabolic trough-shaped mirrors to focus sunlight onto a receiver tube that carries a heat transfer fluid. This fluid is heated up and is pumped through a series of heat exchangers to raise steam for a conventional steam-turbine-driven electric generator. The parabolic trough technology is a well-proven solar thermal electric technology.

Trough System. The text above describes the image.

(Source: This web page has hyperlinks which may transfer you to third-party website.Sandia National Laboratories Energy Photo Database)

(b) Dish/Engine System

The main parts of a dish/engine system are the solar concentrator and the power conversion unit. The concentrator (or the dish) collects the solar energy coming directly from the sun and focuses it on a small area. The dish structure tracks the sun continuously. The power conversion unit consists of two parts, the thermal receiver and engine/generator. The thermal receiver absorbs the concentrated beam of solar energy, converts it to heat either in the form of hot air or hot water, and transfers the heat to the engine/generator. The engine/generator system is the subsystem that takes the heat from the thermal receiver and uses it to produce electricity. The most common type of heat engine used in dish/engine systems is the Stirling engine.

Dish/Engine System. The text above describes the image.
(Above source: This web page has hyperlinks which may transfer you to third-party website.Sandia National Laboratories Energy Photo Database)

(c) Power Tower

This technology uses an array of sun-tracking mirrors (heliostats) to focus sunlight onto a central receiver mounted on top of a tower. A heat transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine-generator to produce electricity. Power towers developed in the 1980's utilized steam as the heat transfer fluid. The more recent power towers use molten nitrate salt as the heat transfer fluid due to its superior energy storage and heat transfer capabilities.

Power Tower. The text above describes the image.
(Source: This web page has hyperlinks which may transfer you to third-party website.Sandia National Laboratories Energy Photo Database)

Small concentrating solar devices

A number of small concentrating solar devices are available in the market. Such devices have not been used in Hong Kong nor tested for suitability for use in Hong Kong. Examples of some of these devices are given below only for illustration purpose. Strictly speaking, some the these devices are not CSP as we defined above, but instead are concentrating PV devices because solar cell instead of heat transfer fluid is used to receive the concentrated sunlight.

Applicability in Hong Kong

The amount of power generated by CSP depends on the direct sunlight at the site. In Hong Kong, a large proportion of the solar radiation is diffuse rather than direct. The annual proportion of the direct beam component of sunlight is less than 50%, which is relatively low, suggesting that solar concentrator technology is unlikely to be effective for Hong Kong.

2) Solar Space Heating

In a solar heating system, energy from the sun is captured to heat up air or water to provide space heating. Solar space heating systems can be classified into either active or passive types.

Active solar space heating

Active solar space heating systems consist of solar collectors and auxiliary pumps, to absorb solar radiation for heat up air or water and to distribute the heat. Energy storage facilities may be included into the systems to store up the heat so as to continue supplying heat in the hours of the day when there is no sunlight.

Active solar heating systems can be divided into two major groups based on the type of fluid used in the solar collectors:

  1. Liquid-based systems use water or an antifreeze solution as the heat-transfer medium in the solar collectors. Liquid systems are more often used when energy storage facilities are included.
  2. Air-based systems use air as the heat-transfer medium in the solar collectors.

Passive solar space heating

Passive solar space heating use the heat from the sunlight directly through design features. These including the application of south-facing windows to introduce more sunlight to the interior space, and the application of building materials that increase heat absorption to the interior space but reduce heat loss to the environment.

Heat from the sun can be introduced to the interior space passively mainly by one of three designs: direct gain, indirect gain and isolated gain.

  • Direct gain design - the building materials (such as tiles and concrete) store and slowly release the heat energy collected from the solar radiation shining into the building.
  • Indirect gain design - uses materials that hold, store, and release heat; the material is located between the sun and living space (typically the wall).
  • Isolated gain design - collects solar energy remote from the location of the primary living area. For example, a sunroom attached to a house collects warmer air that flows naturally to the rest of the house.

An example of a passive solar space heating system is the This web page has hyperlinks which may transfer you to third-party website.SolarWall system (The system is described here as an illustration of a passive solar space heating, even though it has not been installed in Hong Kong.) The SolarWall system is a metal-wall system that uses solar energy to heat and ventilate indoor spaces. It consists of perforated collector panels, ventilation fans and is connected to a HVAC intake. The collector panels are installed several inches from a south facing wall, creating an air cavity. The metal cladding is heated by the solar radiation from the sun, and ventilation fans located at the top of the wall create negative pressure in the air cavity, drawing in the solar heated air through the panel perforations. A connection to an HVAC intake allows air to be preheated before entering the air handler, reducing the load on the conventional heater. This heated fresh air is then distributed into the building through the existing HVAC system or with separate air makeup fans and perforated ducting.

3) Solar Cooling

Instead of heating, heat energy captured in the sunlight can be used to produce cooling. Solar cooling can be done by coupling a solar water heating system with an absorption chiller or an adsorption chiller.

Solar absorption cooling

In an absorption chiller, heat is used to evaporate a refrigerant under pressure from an absorbent/refrigerant mixture. The two most common refrigerant/absorbent mixtures are water/lithium bromide and ammonia/water. Condensation of the evaporated refrigerant provides the cooling effect. Electricity is required to keep the refrigerant running, but the electricity consumption is relatively small compared to conventional electric chillers. Solar absorption cooling systems use hot water from solar thermal collectors as the heat source to evaporate the refrigerant. They are designed to meet the air conditioning load of a building during sunny periods.

Solar adsorption cooling

Solar cooling can also be done by matching a solar water heating system with an adsorption chiller. An adsorption chiller contains water as a refrigerant and silica gel as an adsorbent. The evaporator section cools the chilled water through the action of the refrigerant (water) being evaporated by adsorption of the silica gel in one of the two adsorbent chambers. For a solar adsorption cooling system, the hot water from solar water heating system regenerates the silica gel in the second of the two adsorbent chambers. The water vapor released from the silica gel by the hot water will be condensed in the condenser section which is cooled by the cooling water, such as from a cooling tower. Electricity is required for the controls and for the pumps. But again, the electricity consumption is low compared to conventional electric chillers.

4) Solar Lighting

A solar lighting system channels the outdoor natural sunlight for interior lighting purpose, whenever sunlight is available. Typically, a solar lighting system comprises of a sunlight concentrator, light transmission device (e.g. optical fibers) and an indoor lighting fixture. The sunlight concentrator is mounted outdoors or on the roof of a building to collect sunlight. Its performance can be enhanced by the incorporation of sun-tracking devices or optical concentrating devices. The light collected is then channeled down via the light transmission devices to a lighting fixture. For some types of solar lighting systems, UV and infra-red light filters are installed to filter out UV and to reduce the amount of heat transferred to the indoor environment.

There are still application limitations associated with solar lighting. Light transmission distance is often limited due to attenuation within the light transmission device. Lighting intensity fluctuates with the natural sunlight. Systems are being developed to use artificial lighting to compensate for the fluctuations.

A number of solar lighting devices are available in the market. Most of such devices have not been used in Hong Kong nor tested for suitability for use in Hong Kong. Examples of some of these devices are given below only for illustration purpose:

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