Cool Energy is saying their SolarFlow® System in an average size US home can provide up to 80% of heat, 100% of hot water and 60% of the electricity needed.

Using a logical controller the system uses weather, ambient temperature, building temperature, and sunlight data to determine the device’s most effective usefulness for a given day.  On warm and sunny days, the controller would direct system components to generate electricity.  When it’s cold and overcast outside and the building’s internal temperature is below a given desired temperature, the controller will direct the system to deliver heat to the building.  A structure would likely need an electrical and fuel backup, but the size would be smaller and the demand much lower.  Yet a battery backup and heat store could preclude even that.

This idea should work.

The SolarFlow system development has backing from the National Science Foundation, the Department of Energy, and electricity and natural gas giant Xcel Energy.  The system is a development based on Cool Energy’s SolarHeart engine, a proprietary Stirling engine developed by Cool Energy to efficiently convert solar thermal energy or waste heat into electricity. The company’s lab test statistics over a wide array of temperature and solar conditions show the SolarHeart engine able to generate more than 2,000 watts of electricity, and achieve more than 16 percent efficiency for thermal-to-electrical conversion.

The advantage of controlling the solar energy output to either heat or electricity should allow customers to recoup their investment in the device more quickly compared to solar panels that provide electricity only. This advantage should be especially effective in climates where homeowners need home heating oil or propane to heat their homes in winter, and electricity-guzzling air conditioners to cool their homes in summer.  That covers a very large part of the U.S.  Pair up some ground source for cooling and the system will need very little grid or fuel inputs at all.

SolarFlow Comparison Piecharts. Note the outer ring of energy coverage. Click image for the largest view.

The financial backing is to go for field tests that will offer more insight on this system’s true real-world capabilities.

The components of the SolarFlow® System include the SolarHeart® Engine which incorporates Stirling engine technology, solar thermal collectors, thermal storage, a hot water & space heater, and the SolarSmart™ Controller control system to create the highest value from the system to the owner. Cool Energy’s claim is the SolarFlow System provides the lowest cost of energy (heat and electricity) of any renewable energy system.

SolarFlow Block Diagram. Click image for the largest view.

Simply put, when surplus heat is available from the solar collectors, the system produces electricity.  The system is always looking and feeding the highest cost component of the energy demand.  The field trials will be very interesting, indeed.

It’s easy to understand the payoff during the winter months; most of the energy from the collectors is used to heat the building’s living space. But in the summertime, the system’s engine converts that thermal energy to electricity when it is needed the most. Here’s the next innovation: peak utility electrical loading requirements are better matched with the SolarFlow system’s production than photovoltaic because of the thermal storage feature.  Storage allows extra energy production throughout the day to be used into the night and holds a reserve for sub optimum solar production days.  Granted, 2000 watts is like a 16.6 amp circuit, but in a steady state of providing a home’s own base load power, it will add up.

If the testing in the real world holds up to the lab result a covering of 100% of the hot water, 80% of the space heating and 60% of the electrical demand some clever folks are going to realize that larger panels and more storage will take the heat demand to 100%.  For many using fuel oil, propane and other high cost heating fuels, this kind of project could pay off rather quickly.

The system block diagram looks pretty simple. Even the test rig photo isn’t terribly complex. What will matter is the longevity of the Stirling engine and the assorted valves and pumps.  A cost cutting war, quality control effort and intense value engineering for buyers needs started at this point.

SolarFlow Pilot System Less Solar Panels. Click image for the largest view.

There are a of couple issues, though.  It’s proposed to use evacuated tube solar collectors that need to get to, ready, a minimum stagnation temperature of 250°C (475°F) for optimal engine performance.  That will need some more thought. Even if that comes down, the tube array will need protection, which leads one to think an awning control within the system controller would be a definite need.  The system’s most likely installation range is going to be windy, snow blown and at risk for hail.  An automatic awning or cover will likely be mandatory if insuring the panels is to be economically viable.

This looks good – if the cost gets competitive, the engineering is complete and the longevity can compare favorably.


3 Comments so far

  1. David Martin on January 27, 2011 5:09 AM

    The information given by Cool Flow provides no basis for evaluation:
    ‘SolarFlow® System in an average size US home provides up to:
    80% of heat, 100% of hot water and 60% of electricity

    ‘Up to’ means it could be anything, but even more importantly no data is provided giving the climate regime assumed, as the US has many different climates, or indeed anything which gives a handle on what is going on.
    80% of the heat where? In Minnesota, or Phoenix?

  2. Sam Weaver on January 27, 2011 9:01 AM

    Thanks for the well-written article on our SolarFlow System – we appreciate the coverage and the analysis. A few comments:

    1) Evacuated tube collectors are remarkably robust, and to receive SRCC rating must pass the hail test, which is pretty brutal. The key is the round shape of the glass envelope, which provides good structural strength. In any event, even if some glass envelope tubes are broken, a new envelope tube slips into the installed collector, replacing the broken one.

    2) The high stagnation temperature we suggest is usually found in a class of evacuated tube collectors called u-tube collectors. Many evacuated tube collectors use a water-based heat pipe to transfer heat to the circulating fluid, and these stagnate at a lower temperature, and ultimately perform less well in cold climates than the u-tube style. The higher efficiency in cold weather translates to higher temperatures in the summer, improving our electrical generating efficiency.

    3) As to David Martin’s intelligent comment, it is precisely due to the variability of climate and insolation as well as building insulation, that we have had to boil our performance claim down to a one-liner: up to 80% of home heating, 100% of hot water, and 60% of electricity. For instance, in my home in Boulder, CO, it would be 80% of home heating, 100% of hot water, and 75% of electricity. But I have a well-insulated home and am somewhat frugal with electricity. As we are working with potential pilot partners, we analyze each site individually, using utility bills and local weather and solar data to predict system performance. Areas such as Chicago, Buffalo, NY, and Seattle will perform relatively poorly, and areas such as Eastern Maine, Colorado, and Lake Takoe, often expect performance near our one-liner for a decently-insulated home.

    Again, thanks for your review. Feel free to check our blog at for continuing news on our progress.

    All the best!

  3. Al Fin on January 27, 2011 9:50 AM

    The system should also have a natural gas or propane hookup to be fully functional — for heating backup. This would allow either off grid or on grid functionality.

    Presumably, the system would also be grid inter-tied for most homes for power backup. Unfortunately, electrical power fed to the grid in piddling amounts, at unpredictable times, cumulative from hundreds of thousands of customers, is a headache for grid managers.

    The best use of the system — if it works — is for off grid.

    The best use of the system when grid inter-tied would be to not allow it to feed power to the grid. Any excess heat over and above power requirements should be fed to a molten salt heat accumulator.

    The power grid is a very important commons resource, and should be protected from green saboteurs — including those who want to convert to a “smart grid”, which only makes it more vulnerable.

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