Technology
|
|
What is a heliostat and how is it used? A heliostat is a mirror that tracks the Sun. A heliostat array is a group of heliostats. Practical Solar Heliostats track the Sun in order to reflect its energy onto a fixed target. An array may direct the Sun's energy to a common target, much smaller in size than the summed areas of the reflectors (0.8 m2 reflector area per heliostat). In this case, the Sun's energy is concentrated on the target. Systems that collect and concentrate solar energy are concentrating solar power ("CSP") systems. An example of a non-concentrating application is Natural Lighting (a.k.a. Daylighting). Heliostats can direct sunlight to areas inside buildings that would not ordinarily receive sunlight, such as through north-facing windows, or down into atriums, usually by bouncing sunlight off of a secondary reflector over the atrium. Heliostats have also been used to illuminate outdoor parks previously shadowed by surrounding buildings. How much energy does a Practical Solar Heliostat produce? Short answer: When the sun is shining, each Practical Solar Heliostat is providing 600 watts (0.6 kW) of power and 65,000 lux of visible light. This means that a single Practical Solar Heliostat provides as much thermal energy as a space heater on a medium setting and as much visible light as forty 100-watt light bulbs concentrated on a one-square-meter surface. The amount of thermal energy a Heliostat can produce over a period of a month or a year depends on the sunlight received in that geographical area. For a more detailed explanation of thermal energy produced and how to predict a Heliostat’s output, please continue reading. Long answer: The human eye is designed for sunlight. A common unit measurement of visible light is the lumen. Lux is a measurement of the intensity of lumens spread out over a one-square-meter (1 m2) surface. The average hallway has illuminance of 80 lux, or 80 lumens per square meter. A brightly lit office can have illuminance up to 400 lux. Amazingly, sunlight produces up to 100,000 lux on a clear day, equivalent to the visible light that would be produced by sixty 100-watt light bulbs, if all of the light were concentrated on a 1 m2 surface. Energy radiated by the Sun is also known as insolation, often expressed in kilowatts per square meter (kW/m2). Insolation is the sum of direct insolation, meaning sunlight coming straight from the Sun to the surface, and indirect or diffuse insolation, meaning sunlight reflected by other surfaces and molecules in the air to arrive at the surface from all angles. On a cloudy day, one might not see the Sun at all, but the region is still fully illuminated by indirect insolation. Over the course of a year, the percentage of direct vs. indirect insolation varies by region, but a good rule of thumb is that about 2/3 is direct insolation and 1/3 is indirect. The intensity of insolation per square meter depends on the latitude, the weather, the season of the year, and the time of day. These variables account for large differences in available insolation at any given moment in different locations around the world. Despite the differences, insolation on a 1 m2 surface perpendicular to the Sun anywhere on Earth receives approximately 1000 watts (1 kW) of solar energy on a clear day. This figure of 1000 watts represents "peak" power or maximum power, undiminished by fog, smog or cloud cover. It is the equivalent of the output of an average space heater or hair dryer. Thus a 1 m2 window or skylight perpendicular to the bright Sun admits about 1kW of energy to the space inside. This would be more obvious if the energy was concentrated, as it is when coming out of a space heater. But the total amount of energy is the same; it has the same impact on the temperature of the space inside. A Heliostat with a 1 m2 mirror likewise receives 1 kW (“NORMAL” TO SUN) of energy from the Sun. The total of amount of energy successfully redirected to the target is diminished by three factors: (1) the imperfect reflectivity of the mirror, i.e., mirrors reflect less than 100% of the sunlight striking the surface - some of the energy is absorbed, and (2) only direct insolation will be reflected by the Heliostat to the target; indirect insolation will be reflected at all angles and "miss" the target, and (3) the "cosine error", meaning the degree to which the Heliostat deviates from perpendicular to bisect the angle between the Sun and the target. The amount of insolation collected at the target(s) is also a function of the number of Heliostats in the array as well as the size of the mirrors. Thus a 50-Heliostat array will produce approximately five times more energy than a 10-Heliostat array, assuming all mirrors are the same size. The United States government has compiled comprehensive statistics on insolation intensity at hundreds of locations. See http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/. These tables provide an easy and quick reference to determine the amount of insolation per square meter (expressed in kilowatt hours per square meter: kWh/m2) one might expect during an average day, month, and year usable by a CSP system. Most locations in the United States receive enough insolation year-round to generate considerable energy from a CSP system. One can obtain an estimate of the energy output of a Heliostat array by performing the following simple calculations:
Example: Assume the array has 25 Heliostats, each with 0.8 m2 mirrors, for a total reflector area of 20 m2, and the expected insolation locally for a two-axis tracking system is 5.0 kWH/m2/day. This array would provide 20 m2 x 5.0 kWh/m2/day x 0.8 = 80 kWh of solar energy per day. In bright sunlight the system would have peak output of 20 m2 x 1 KW/m2 x 0.8 = 16 kW. This is the equivalent of the peak output of 16 typical space heaters or a 54,500-BTU furnace. The array would have expected annual output of (80 kWh/day) x (365 days/year) = 29,200 kWh/year or (29,200 kWh/year) x (3,412 BTUs/kWh) = 99,630,400 BTUs/year. This happens to be the amount of energy consumed annually by an average American household. Practical Solar Heliostats vs. photovoltaics (PV) and thermal flat plate collectors In traditional solar energy systems (e.g. fixed-position solar thermal systems, photovoltaic panels) installed in the Northern Hemisphere, the collectors are generally pointed due south and tilted at an angle to the ground equal to the latitude of the installation (e.g. 0° is horizontal). This position represents a compromise: by facing due south, each collector gathers maximum sunlight at midday but still receives some sunlight in the morning and late in the day. However, the angle early and late in the day becomes increasing acute, so that the collector becomes, in effect, a smaller target. This is known as the cosine error. Likewise, the tilt of the collector at latitude is a compromise. If optimized for the spring and autumn solstices, it will collect less solar energy in winter when the Sun is lower in the sky, and less solar energy in the summer when the Sun is higher in the sky. A Heliostat system largely eliminates the cosine error because the Heliostats rotate left/right (x-axis) and up/down (y-axis) to precisely track the movement of the Sun during the course of the day and reflect its energy to the north-facing receiver. Thus a line perpendicular to the Heliostat mirrors bisects the angle between the Sun and the receiver. The cosign error for such milder angles is small, especially during peak collection periods during the middle of the day (typically less than 5%). CSP systems have other inherent advantages over fixed-position thermal flat plate collectors. The fluid or gas running through the thermal collector (the heat transfer medium) must arrive to the point of use at higher temperature than when it departed or else, by definition, there is no thermal gain. Most thermal losses occur across the front surface of the collector. Thermal losses increase in direct proportion to the size of the front surface area. The difficulty is that the amount of energy received over the surface area of the collector may not offset thermal losses resulting from its surface area in colder weather. In a Practical Solar Heliostat System, the Heliostats are not the collectors themselves; rather they reflect the solar energy to a single collector. Therefore, a Heliostat System can deliver many times the amount of solar energy to the same size surface area as a single Heliostat. This ensures that the temperature of the heat transfer medium is raised to sufficiently high levels even in sub-freezing weather conditions. Temperature sensors can provide feedback to flow control valves and pumps to maintain temperature at desired levels. Practical Solar is in discussion with several companies about design of such a CSP receiver, designed for extremely high solar input with failsafe mechanisms to prevent damage in the event of a power shutdown. How can a Heliostat’s energy output be stored? Thermal storage is highly desirable if more thermal energy is provided than consumed at certain times of the day. Thermal storage is accomplished by moving the heat-transfer medium through an insulated vessel or tank where thermal energy is dissipated into the vessel. Combinations of water, pebbles and/or stone are commonly used for thermal storage. Practical Solar Heliostats combined with concentrator cell technology (advanced PV) Another promising aspect of CSP is its compatibility with advanced photovoltaic (PV) or concentrator cell technology. Traditional PV systems are engineered to convert solar energy to electricity based on an input from "one Sun", i.e. non-concentrated insolation. The performance of PV degrades rapidly as ambient temperatures rise above 35°C (95°F). This accounts for why PV systems installed in tropical climates must typically be 25% larger to achieve the same output as in temperate climates. Concentrator cells are capable of accepting 50-1000 Suns (i.e. sunlight multiplied 100-1000 times) with linear output of electrical power. Some concentrator cells operate at higher temperature, and most at higher efficiency. While most traditional PV can achieve a peak solar-to-electric conversion efficiency of about 12%, concentrator cells have achieved 20-40% efficiency. Cells under development are expected to have even higher efficiency. Practical Solar is evaluating the possibility of engineering a new receiver combining advanced PV technology to convert solar energy to electricity and use excess thermal energy from the receiver for productive purposes. The heat-transfer medium running through the receiver would serve the dual purposes of cooling the receiver to desired levels and transporting the thermal energy to the point of use. The principle at work here is combined heat and power ("CHP"), commonly employed for turbine generators where "waste heat" is collected and used in other applications. Recent developments in thermo-electric ("TE" technology) also present fascinating potential for CSP. The Practical Solar Heliostat system is a small-scale version of "power tower" systems. See the Department of Energy (DOE) Sun*Lab website www.energylan.sandia.gov/sunlab/sunlab.htm for an overview of large-scale CSP systems. More information - useful links The following websites offer some interesting information on solar energy and concentrating solar power: www.ases.org/ [website of American Solar Energy Society] www.csemag.com/article/CA6501326.html?industryid=47249 [Article "Light Untamed", providing an overview of natural lighting technical issues ] www.distributed-generation.com/ [informational site sponsored by consulting company specializing in distributed generation] www.eere.energy.gov/solar/ [Department of Energy solar program] www.eia.doe.gov/cneaf/solar.renewables/page/solarthermal/solarthermal.html [DOE website on solar thermal systems and history] www.freepower.co.uk/ [UK manufacturer of Organic Rankine Cycle (ORC) microturbines] www.fsec.ucf.edu/ [Florida Solar Energy Center, affiliated with the University of Central Florida] www.mtpc.org/RenewableEnergy/index.htm [Renewable Energy Home Page for the Massachusetts Technology Collaborative] www.nrel.gov/ [National Renewable Energy Laboratory] www.nesea.org [Northeast Sustainable Energy Association] www.solarbuzz.com/ [solar information website] www.solarpaces.org [website for CSP Program of International Energy Agency] www.sterlingplanet.com [website promoting renewable energy] www.raycotechnologies.org [super-efficient New England home & Practical Solar beta test site] |
E-mail info@practicalsolar.com with questions or comments about this web site. © Practical Solar, Inc. 2008. All Rights Reserved. Last modified: 11/20/08. |