Our Technology

Practical Solar heliostats are the most reliable, cost effective and highest performing heliostats manufactured anywhere in the world. Practical Solar is the only company that can realistically show its heliostats will reliably run without failure for 30 years.

How Practical Solar's heliostat system can be used

HVAC Systems: Installed on the flat roof of a commercial building, Practical Solar heliostats are designed to supply all of the building's heating and cooling needs. For a number of reasons, solar panels and solar hot water systems are not suited for this task. Practical Solar heliostats collect and process solar energy in a different way, allowing them to do the job of heating and cooling commercial buildings 365 days a year without fossil fuels. The system is designed to pay for itself in two to six years. A large reservoir of water brought to near boiling temperature can inexpensively store enough energy to supply all the HVAC requirements of a commercial flat-roofed building for weeks in New England in the dead of winter. Practical energy storage overcomes the primary complaint leveled against solar energy.

NASA Testing: In 2012, Practical Solar entered into a limited manufacturing agreement with EMCORE Corporation, an Albuquerque-based manufacturer of space-qualified photovoltaic cells and panels. EMCORE received a NASA contract to provide solar panels for the Solar Probe+ mission to the outer atmosphere of the sun. The panels needed to be tested in as close to actual flight conditions as possible. Practical Solar granted EMCORE a manufacturing license to build up to 200 Practical Solar HE-500 heliostats. Practical Solar also supplied a custom version of its computer control system.

Other Applications: Practical Solar heliostat systems can be used for a wide variety of applications requiring either sunlight or thermal energy, from illuminating a Northern European town shadowed by mountains, to providing the heat for a commercial beer or wine operation. Other applications include:

  • Water desalinization
  • Electricity generation
  • Residential heating and cooling
  • Contaminated soil remediation
  • Agricultural drying and curing
  • Milk pasteurization
  • Increasing greenhouse productivity
  • Melting snow and ice
  • Providing sunlight to dark spaces

Read more:

How Practical Solar's heliostat system works
Why Practical Solar's heliostat system is unique
How other solar technologies compare
Reliability of Practical Solar's heliostat system
Safety of Practical Solar's heliostat system

How Practical Solar's heliostat system works

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.

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Why Practical Solar's heliostat system is unique

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:

Solar power tower

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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How other solar technologies compare

Photovoltaic panels

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 system

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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Reliability of Practical Solar's heliostat system

Because heliostats contain moving parts, operate completely exposed to the natural elements and are expected to operate for decades with only minimal maintenance, it is fair to question long term reliability. Below is a description of our heliostat technology as it relates to reliability and calculation of mean time between failure (MTBF).

Twenty-five heliostats were built in 2008 and subjected to accelerated life testing of the moving wear parts and MBTF of the electronics. Practical Solar heliostats incorporate powdered steel technology gear trains. These gears are very rugged and low cost, costing only about 10 percent that of machined gears. Because the sun travels across the sky so slowly and isn’t out at night, the duty cycle of the gear drive system is less than 0.5 percent. Therefore it is possible to greatly accelerate wear testing of the gear drive by running them at close to 100 percent, allowing the equivalent of decades’ worth of wear to be simulated in only weeks and months. Continuous testing of the gear train took place over a period of two years, where many combinations of gear shafting material and lubricants were evaluated. We have determined that there is negligible gear wear over the equivalent of 100 years of normal operation. Essentially, gear life is infinite in this application.

Practical Solar heliostat gear trains are driven by small brushed DC motors (two per heliostat) manufactured by Mabuchi Motors in Japan. Mabuchi Motors is the largest motor manufacturer in the world. Every car in the world uses dozens of Mabuchi DC motors. We set up special life test fixtures to accelerate testing of these motors by many times. Many motors were tested simultaneously, from different production lots, and also manufactured in different factories both in Japan and China. The motors were run under as close to actual real life conditions as possible, and also under much heavier loads and higher speeds than real world conditions. The motors were then taken apart and inspected for brush, commutator and bearing wear. We found that after the equivalent of 20 years of normal operation the carbon brushes had only lost about 15 percent of their length, the commutator was only slightly worn, and the sintered bronze sleeve bearings showed only slight wear. Not a single motor out of hundreds tested ever failed.

Because of the extreme gear reduction from the motor to the mirror drive shafts, the torque that can be produced is huge. If the mirror support frame is mechanically prevented from moving, as it might if buried in snow, the torque generated could be enough to snap teeth in the final gear in the gear train. The electronic motor drive circuitry includes electronic torque limiting, eliminating this potential reliability problem.

As with almost all modern electronic circuitry, Practical Solar heliostats use a mix of analog and digital circuitry. The parts count is quite modest, and all components are rated over the industrial temperature range. Capacitors, resistors, diodes and transistors are hugely current and voltage derated. Multilayer boards have been avoided to eliminate interlayer via failures.

The failure of a single heliostat in a field of a hundred heliostats only reduces the energy output of the field by 1 percent. If a heliostat field failure is defined as an energy output decline of 5 percent, then five heliostats would need to fail. If heliostats fail at random times and are generally quite reliable (high MTBF) then the effective MTBF of the entire field is much higher than the MTBF of the individual heliostats. With ongoing and continuous accelerated life testing, an MTBF of many decades of continuous operation can be achieved in a heliostat array of many individual Practical Solar heliostats. A MTBF of 50,000 hours for a deployed heliostat array in commercial building HVAC applications is a conservative estimate based on past life testing.

Going forward, continuous stress testing will be part of our reliability assurance. Every mechanical, electronic and software subsystem will be subjected to continuous temperature, humidity and vibration stress testing to seek out subtle weaknesses. We believe that field failures can be brought to near zero over the expected 30-year minimum life of the heliostat array.

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Safety of Practical Solar's heliostat system

Flat-roofed commercial buildings in almost every geographic area of the United States receive sufficient solar energy on the roof alone to supply all the heating and cooling needs of the building. But solar concentration is required to achieve sufficient efficiency. Heliostats can concentrate sunlight to almost arbitrarily high values cheaply and reliably. These high values of solar concentration have raised questions of safety.

A flat-roofed building with a surface area of 10,000 square meters would be covered with an array of several hundred Practical Solar heliostats. These heliostats, under computer control, will reflect over 80 percent of the sunlight falling on the roof to a single target (thermal receiver) of less than 2 square meters in surface area, a potential concentration of several thousand times. If the thermal receiver wasn’t actively cooled with water, it would melt or catch fire regardless of the materials used in its construction.

Relatively cold water enters into the thermal receiver, flows through a set of baffles, and then exits as hot water. Hot water is the product; heating the building in the winter and cooling it in the summer using well-developed absorption chiller technology. The system is designed to heat water close to the boiling point but not create steam, so the system isn’t pressurized to any extent. Flow rate, pressure and temperature sensors are integral to the thermal receiver. If the control computer reading these sensors detects a fault condition, then the heliostats are commanded to move off the thermal receiver. The time between detecting a fault condition and all heliostats responding to the computer command is only a few seconds, short enough so that the thermal receiver won’t be damaged. For example; the main water pump fails. The computer detects a decrease in flow rate. Within a few seconds, an increase in temperature is detected, and a few seconds after that a pressure increase is detected due to water turning to steam. The thermal mass of the thermal receiver is sized such that the heliostats are moved before any significant pressure rise can damage the thermal receiver.

The control system is designed to handle routine failures of pumps and sensors. The various pumps and the building’s HVAC system are powered by the building’s utility electric service. The heliostats and computer control system are powered by an independent power source in the form of a single 150W solar panel charging a 24 VDC lead-acid battery. Therefore, any failure in the building’s HVAC system or a total loss in AC power will trigger an automatic shutdown of the heliostat array.

A remote but far more difficult safety challenge is the failure of the computer controlling the array or other catastrophic failure that causes the heliostats to simply stop moving AND a simultaneous failure of the main water pump cooling the thermal receiver. In such a case the concentrated sunlight will continue to heat the thermal receiver until the sun movement across the sky moves the concentrated spot off the thermal receiver. This would take several minutes. (As a slight aside; because the thermal receiver generally is located south of the heliostat array and the sun travels east to west, the highly concentrated spot would move right to left off the thermal receiver. The size of the spot would increase and concentration decrease hugely within 10 minutes.) It has been determined that it simply isn’t practical to build a thermal receiver that is sufficiently robust to survive such an event; therefore the thermal receiver is designed to be sacrificial. If all cooling was lost, the water present in the thermal receiver would turn to steam, internal pressure would increase until a pressure safety vent opens. All the water inside the thermal receiver would be lost and temperatures would then rise quickly until the aluminum frame and front surface melted. A steel tray located several feet under the thermal receiver would collect the melted aluminum. The damage would be limited to the thermal receiver. Such a failure would result in a few thousand dollars of materials and labor.

There has been increasing concern that computer hackers could gain access to the heliostat array control system and cause havoc. Gaining access to an Internet-connected operating system is certainly possible; even a well-protected one. But gaining control of the array to concentrate onto another target, not initially programmed during field setup, turns out to be a mathematically intractable problem. It’s fairly simple to show that the number of unknowns exceeds the number of knowns in calculating the required position of the individual heliostats. In particular, nowhere in the heliostat data files accessed by the control program is the distance from a particular heliostat to the thermal receiver. Each heliostat only “knows” the angular position (azimuth and altitude) of the target not its physical distance (could be 20 or 200 meters). The hacker might know the location of his desired target but cannot calculate the required command to the individual heliostat without knowing the heliostats relative distance to that target. One might argue that the hacker could climb the roof, survey the entire heliostat array, do the spherical trigonometry and disassemble the code. There are easier ways to burn a building down. Purposely attempting to interfere with airplanes flying above the heliostat array would face the same problems, and additionally that the heliostats move far too slowly to track aircraft.

Not just the surface of the thermal receiver is subject to very high solar concentration; the immediate volume around it is too. Birds and people are the primary concern. It turns out this isn’t a big problem. Solar concentration drops quickly as the distance from the thermal receiver increases. A good analogy is the decrease in concentration of a magnifying lens as it’s moved forward or back from its focal point. Our heliostat array is best modeled as a very short focal length lens. (The ratio of the focal length to diameter is about equal to one.) The thermal receiver is mounted at least 10 feet above the roof, so anyone walking underneath will be safe. Birds will be prevented from flying into the dangerously high solar concentration volume with chicken wire.

In conclusion, it could be argued the actual danger of operating a heliostat array to provide the HVAC needs of a commercial building is less than operating a conventional HVAC system. The heliostat system runs at a temperature no hotter than boiling water, runs at low pressure and there is no use of flammable fuels.

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