Reflecting energy: A Practical Solar heliostat is a mirror -- not a solar panel or other complex material – that makes precise movements up/down and left/right to reflect sunlight onto a fixed spot. As the sun marches across the sky, the heliostat adjusts its position, so the spot of reflected light remains stationary on the target. All of the complex mathematical calculations required to position the heliostats are left to Practical Solar’s proprietary software. The position of the sun in the sky is calculated using an astronomical algorithm. If the latitude, longitude, date, and time of day are known accurately, the position of the sun can be calculated exactly. Even small errors such as the refraction of the atmosphere near the horizon, or the tilt of the pole on which the heliostat is mounted, are compensated for in the software. The relative spherical angular position of the thermal receiver to the heliostat is inputted to the computer. The computer solves the spherical trigonometry problem and commands the heliostat motor drive system such that the mirror is positioned angularly exactly halfway between the sun and the thermal receiver. Because every heliostat in an installation is in a unique position relative to the thermal receiver, each heliostat receives unique position commands.
Concentrating energy: When multiple Practical Solar heliostats reflect sunlight onto a single thermal receiver, the concentrated heat of the sunlight can be used to produce hot water or steam. Relatively cold water flows through the thermal receiver and is outputted as hot water. Although such a system can generate temperatures capable of melting steel, the temperature of the water is raised to just within a degree of boiling. For the purpose of heating and cooling a commercial building, this temperature uses the heat in sunlight at the highest efficiency and lowest cost. A higher temperature would mean more heat loss, as well as a more expensive system to withstand the higher temperature and pressure.
The mirrors on Practical Solar heliostats have minimal reflection loss, so each heliostat reflects approximately its area in sunlight: about 1 kilowatt of heat per square meter. As a frame of reference, a typical electric space heater produces 1.5 kilowatts of heat. If one hundred Practical Solar heliostats, each with 2.2 square meters of mirror area, direct sunlight onto a single thermal receiver, the sunlight will be converted into 220 kilowatts of heat.
Storing and distributing energy: With Practical Solar’s method of heat storage and heat distribution, 220 kilowatts is more than enough energy to supply all of the heating needs of a 10,000 square foot commercial building. Using absorption chillers powered by hot water, it is also sufficient energy to supply the building’s cooling needs.
However, the sun never shines at night, and often does not shine during the day. Energy storage is a fundamental requirement in any serious solar energy application. It is extremely expensive to store electrical energy, even though billions of dollars have been spent improving battery and fuel cell technology. The reverse is true of thermal energy. Hot water can be stored cheaply in a thermally insulated tank. As the volume of water and energy stored increases, the cost and losses of thermal energy storage drop rapidly.
The HVAC requirements of a typical 20,000 square foot, single story commercial building in the Boston area can be stored in a cube of water approximately 8.5 feet on a side for one day of HVAC needs, 17 feet on a side for one week of HVAC needs, and 27 feet on a side for 1 month of HVAC needs.
The stored thermal energy acts as the boiler, but is larger than a conventional boiler and is charged by the heliostat array rather than fossil fuel. From the boiler on, the method of heat distribution is a conventional commercial HVAC system, using forced hot water or air to provide heating. Absorption chilling provides air conditioning.
Take a small mirror outside and position it so it reflects sunlight to a desired target. Pretty easy, right? It is extremely challenging to make a machine to do this same seemingly simple task. To also do this task at low cost, and requiring little or no maintenance for decades, is devilishly difficult. At present, Practical Solar’s heliostats combine the highest known levels of accuracy, reliability, and affordability available in heliostat technology. This combination of features makes it the only solar product that can compete against fossil fuels for heating and cooling commercial buildings.
Practical Solar’s heliostat is a reality because of advancements in several technologies. Twenty years ago, it would not have been possible to manufacture this heliostat at a reasonable cost. The price of structural materials, such as steel and concrete, has steadily risen over the past two decades, but highly automated computer-controlled machining centers, and new engineering materials and processes have dramatically lowered the cost of small precision mechanical parts. The cost of computer processing power has declined by orders of magnitude over the last two decades.
The following are some of the important manufacturing advances Practical Solar has brought to its heliostat design:
Size: Practical Solar heliostats are tiny in comparison to almost all previous manifestations of this technology, including commercial products and government projects. Smaller heliostats are better suited to roof mounting because they are far more rugged, stress the roof structure less, and are typically not visible from ground level. A government concentrating solar project in the 1980s and 90s used heliostats that were huge in size – as large as a house (see photo). At that time, it made sense to build a smaller number of giant heliostats, rather than a large number of smaller heliostats, because the cost of the control electronics, computers, and small precision mechanisms was high. As the cost of such items has plummeted over the last twenty years, it makes increasingly good sense to build smaller heliostats.
Accuracy: The mirrors on Practical Solar’s heliostats need to be positioned up/down and left/right to continuously reflect sunlight to the thermal receiver, as the sun moves through the sky. The Practical Solar heliostat design employs two proprietary high-accuracy rotary position encoders that allow the reflected beam of sunlight to be positioned to within three inches, at a distance of one hundred feet from the receiver. Encoders with such accuracy commonly cost hundreds of dollars each. Practical Solar has reduced the manufacturing cost to less than ten dollars per axis.
Reliability: Great effort went into designing a drive system that is rugged, extremely low wear, low power and inexpensive. Practical Solar heliostats use small DC motors to drive powdered metal steel gears with a total gear ratio of many thousands to one. This drive system has been subjected to accelerated life testing for the equivalent of twenty years of operation and shows very little wear. Costs and power consumption have been reduced to less than a tenth that of comparable gearbox designs with similar performance.
Control: The Practical Solar heliostat employs two classes of computers: (1) embedded micro-controllers inside each heliostat that control a myriad of low level tasks, and (2) a computer in the commercial building that runs Windows-based software capable of controlling the motion of thousands of individual heliostats.
At a cost of under a dollar, imbedded micro-controllers run complex software (firmware) that perform many functions including communicating with the central computer, controlling the motor drive system and position encoders, correcting for errors due to temperature and time, and monitoring the health of the heliostat. This computer code cannot be read out or copied even by the micro-controller manufacturer, making our heliostat almost impossible to reverse engineer.
The high-level software that controls the overall heliostat array runs on all Microsoft operating systems, from Windows 95 to present day Windows 7. It will run on the slowest PC without a noticeable reduction in speed. In almost all commercial HVAC applications, the PC will have internet access. This allows the entire system to be monitored, and if desired, controlled from a remote location.
Elegance: Communication between the single Windows based computer and the heliostat array is done over a single wire that also supplies the electrical power to the heliostats. Individual heliostats in an array of heliostats can be connected together in any order because each heliostat has a unique name and location known to the software. Because each individual heliostat requires so little electrical power, the size of the wire is quite modest. A typical array of 100 heliostats requires a single 18 AWG twisted pair cable and consumes 20 Watts of electricity, while delivering 200,000 Watts of thermal power to a central receiver. A single small photovoltaic panel can easily supply all the electrical needs of the heliostat array.
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Solar panels: Traditional photovoltaic solar panels are “passive”, meaning they have no moving parts and do not track the sun. Solar panels convert sunlight to electricity with an average efficiency of 7-12 percent. Practical Solar’s heliostat system concentrates the raw heat already in sunlight and distributes it through a building – in the form of heating or cooling – with an average efficiency of 85-90 percent. The products of these systems are different – electricity versus thermal energy. Electricity can easily be used for heating and cooling, but due to the technology’s comparatively low efficiency, the number of solar panels that would fit on the flat rooftop of a commercial building would not be able to supply the building’s heating and cooling needs. In addition, the solar panels would cost 10 times more than the entire heliostat installation.
Solar hot water systems: Traditional solar hot water systems are also passive. Simply put, these systems cannot achieve the high temperatures and high efficiency possible with heliostats. Solar hot water systems are unable produce hot water in cold weather, making it impossible for them to meet the heating needs of a commercial building.
Heat loss from any surface is proportional to its surface area. Heliostats offer high efficiency because they concentrate the energy of many suns onto a small surface area. In solar hot water systems, the sunlight collector and thermal receiver are effectively one in the same, so the surface area – and heat loss – increase in direct proportion to the amount of energy collected. A Practical Solar installation designed to collect 100 square meters of solar energy (about 45 heliostats) could direct all of the energy – about 100 kilowatts – onto a thermal receiver about one square meter in size. A solar hot water system would require a surface area of the full 100 square meters. The thermal energy loss in the heliostat system would be a hundredth that of the passive system.
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Practical Solar decided to focus on heating and cooling commercial buildings after determining that the payback time for an installation of Practical Solar heliostats in HVAC applications is just 2 to 6 years. But industrial low to mid temperature applications (100-500°F) also have very attractive payback times. These applications, in food and beverage, chemical, cleaning, dyeing, and other industries, are clearly near-term markets for this technology.
Further down the road, there are a number of additional applications that are extremely well suited to a heliostat solution. Some of the more important ones are listed below.
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