Distributed concentrator for solar power generation.

Ciupe, Valentin ; Dehelean, Nicolae ; Maniu, Inocentiu 等

1. INTRODUCTION

The use of solar generated thermal and electric power is an every increasing topic of interest worldwide. The two main areas of interest for electric power are photovoltaics and solar-thermal-electric. So far, the last method gives better results regarding efficiency and cost-effectivness, by using concentrator systems.

Unlike traditional power plants, concentrating solar power systems provide an environmentally benign source of energy, produce virtually no emissions, and consume no fuel other than sunlight. About the only impact concentrating solar power plants have on the environment is land use. Although the amount of land a concentrating solar power plant occupies is larger than that of a fossil fuel plant, both types of plants use about the same amount of land because fossil fuel plants use additional land for mining and exploration as well as road building to reach the mines (NREL, 2001).

In order to determine if a solar concentrator system is feasible in the region of interest, one must check the charts regarding the yearly average insolation and also the average number of clear-sky days, as concentrator systems can only use direct sunlight in order to function.

Figure 1 shows the yearly sum of global insolation, according to the EC's Joint Research Centre (PVGIS, 2008) for Europe and particulary for Romania (the region of interest). The map on the right shows a variation from 1700 kWh/[m.sup.2] to 1300 kWh/[m.sup.2].

Another factor to be taken into account beyond insolation is the availability of solar radiation in the area. This represents the average number of clear-sky days and the average number of diffuse light days (figure 2). The two maps show for the month of July about 75-80% clear-sky days, and less than 40% of diffuse light days.

Considering the paragraphs above, solar-thermal and solarthermal-electric can be used in the region and allows for a fairly efficient systems

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2. CURRENT SOLAR CONCENTRATORS

There are four main types of concentrating solar power systems: parabolic troughs, dish/engine systems, central receiver systems and compact linear Fresnel reflectors. These technologies can be used to generate electricity for a variety of applications, ranging from remote power systems as small as a few kilowatts (kW) up to grid-connected applications of 200-350 megawatts (MW) or more. A concentrating solar power system that produces 350 MW of electricity displaces the energy equivalent of 2.3 million barrels of oil. (NREL, 2001)

Trough Systems solar collectors use mirrored parabolic troughs to focus the sun's energy to a fluid-carrying receiver tube located at the focal point of a parabolically curved trough reflector as shown in figure 3, left (Elrod, 2007). The energy from the sun sent to the tube heats oil flowing through the tube, and the heat energy is then used to generate electricity in a conventional steam generator.

Dish systems use dish-shaped parabolic mirrors as reflectors to concentrate and focus the sun's rays onto a receiver, which is mounted above the dish at the dish center. A dish/engine system is a standalone unit composed primarily of a collector, a receiver, and an engine (figure 3, right). It works by collecting and concentrating the sun's energy with a dish-shaped surface onto a receiver that absorbs the energy and transfers it to the heat engine, usually a Stirling engine.

Central receivers (or power towers) use thousands of individual sun-tracking mirrors called "heliostats" to reflect solar energy onto a receiver located on top of a tall tower (figure 4, left). The receiver collects the sun's heat in a heat-transfer fluid (molten salt) that flows through the receiver. The salt's heat energy is then used to make steam to generate electricity in a conventional steam generator, located at the foot of the tower.

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Compact Linear Fresnel Reflector (CLFR) use flat reflectors (instead of parabolic mirrors), that move on a single axis and Fresnel lenses to concentrate the solar thermal energy in collectors (figure 4, right). The advantage of CLFR is that it allows for a greater density of reflectors in the array and flat mirrors are much cheaper than parabolic ones (CLFR, 2008).

3. THE DISTRIBUTED CONCENTRATOR

Because this level of efficiency is not currently possible or at least economically feasible (in the case of concentrated photo-volt[[alpha].sub.ic]s) a new technical solution must be found.

The starting point is represented by a solution consisting in a large Fresnel acrylic lens, mounted on a pivoting frame which allows it to track the sun during the day while keeping the focused spot approximately in the same place. In the focal plane of the lens the hot cylinder of a modified alfa-type Stirling engine is placed. The engine remains fixed during lens movement.

The optical quality acrylic is the most widely applicable material for a concentrator Fresnel lens, and is a good general-purpose material in the visible. Its transmittance is nearly flat and almost 92% from the ultraviolet to the near infrared between 0.2 [micro]m and 2.2 [micro]m (Fresnel, 2003). This first solution seems to be the easy way from the constructor's point of view but it has some drawbacks: due to the large area of the lens, the focal distance is also long, so the resulting height of the system will be large rendering it less desirable. Another concern is represented by the wind problem, which, considering the surface of the lens becomes quite difficult to solve elegantly.

The second approach is to split the large collecting area into several smaller concentrators arranged either as a strip or as a matrix. Each individual concentrator consists of a smaller Fresnel lens with a lot shorter fcal length monted on a pivoting frame to allow for sun tracking. The smaller lenses are more available and less costly than big lenses (consider decomissioned overhead projector lenses). The focused spot from the lens falls onto the end of a copper pipe containing a thermal agent. The end of the pipe is hemispherical having the diameter larger than the focused spot. This receptor head is blackened in order to collect as much heat as possible and the rest of the pipe is insulated. The other end of the pipe is bolted onto the head of the hot cylinder of the Stirling engine, along with other collector pipes in the matrix (figure 5).

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The heated end of the pipe is hose coupled to the rest of the pipe to allow the movement of the head with the focus point of the lens durind tracking. Following this strategy, system scalability is provided by adding or removing lenses to the matrix while keeping the same engine.

The proposed method for the tracking is based on heliostat operation, or active sun tracking (Red Rock, 2008). The logic modules also present themselves as a descentralized system, each concentrator assembly having its own sensors, motors and control module. These can communicate over a bus and exchange information with a master controller which can also monitor the electrical energy generation and for strong winds or generally bad weather, it can park the concentrators (figure 6).

4. FUTURE RESEARCH

The proposed solution for the sun concentrator must be further optimized and tested in real life conditions.

The next step presumes building a working model of the concentrator and testing the ability to orient and to transport heat.

The implementing of these concentrators will be benefic for remote locations like holiday houses where they can be used as heat collectors for electricity generators or as water heaters.

The downside of the proposed solution could be reciprocal shading of the lenses at dusk and dawn. Another problem that needs attention is lens cleaning if dust or dirt represents a problem in the installation area.

5. CONCLUSION

The proposed system can be used in the area of interst mainly as a power generator for remote locations (with power output up to a few hundred watts) or as grid connected application in plain field or hill areas that are not usable for agricultural purposes. Providing the building and maintenance costs are kept low combined with a long lifespan through system optimisations, it can prove itself effective and gain popularity in the area.

6. REFERENCES

Compact Linear Fresnel Reflector. (2008), Available from: http://jcwinnie.biz/wordpress/?p=2470, Accessed: 2008-06-09

Elrod, S. (2007), Solar Concentrators. Available from: http://www.parc.com/research/publications/files/5706.pdf, Accessed: 2008-06-11

Fresneltech. (2003). Fresnele Lenses product guide, Available from: http://www.fresneltech.com, Accessed: 2008-05-05

National Renewable Energy Laboratory (NREL), US D.O.E. (2001). Concentrating Solar Power: Energy from Mirrors, Available form: http://www.nrel.gov/docs/fy01osti/28751. pdf, Accessed: 2008-05-05

PVGIS Project, (2008) EC's Joint Research Centre. Available from: http://re.jrc.ec.europa.eu/pvgis/, Accessed: 200805-02

Red Rock Energy. (2008) Solar Power FHoslat Arrays. Available from: http://www.redrok.com/main.htm,Accessed:2008-03-21