Clean Power

Published on December 29th, 2012 | by Guest Contributor


Using Thermodynamics & 100-Year-Old Technology To Break The $20 Per MWh Barrier

December 29th, 2012 by  

This is a guest post by one of our regular, cleantech-obsessed readers, David Fuchs. Clearly, David thinks he’s on to something big. Enjoy the article!

For years, the production of energy has fascinated me. Over the past 20 years, I have experimented with solar cells made via inkjet printer, a hydraulically coupled compressor and turbine based on Tesla’s turbine, vertical wind turbines, high-temperature cracking of water, high COP heat pumps, all the different varieties of Stirling engines, and many other energy projects. Continuously going back to old projects to incrementally improve them and make them perfect has been fun, except perfect is the enemy of finished.

The week long power outage here in New Jersey, after hurricane Sandy, made me realize that we need simple, scalable, cheap, and locally produced power. Removing all distractions and giving an engineer of German lineage a week to think on a problem often gets the problem solved. After pulling out the 7-pocket expanding file with all my past Stirling designs, a couple notepads, my favorite gel pens, a dry erase board, and some reference books, I began designing. As with any engineering project, you need to describe what you want to accomplish, and your limiting factors. Due to cost constraints, engineering is always compromise.

What is the goal? An always-on (24 x 7 x 365) power supply that is inexpensive to produce, can be bulk produced with readily available materials, can be manufactured in any nation using 1950′s or earlier technology, and has a working lifespan greater than 20 years. (That sounds really simple, doesn’t it?)

What are the design criteria?

  • Low Temperature Differential (LTD) Stirling based design.
  • All parts must be designed for high-speed manufacture and assembly.
  • All materials used must be inexpensive and readily available.
  • The Stirling design must have the least number of wear points possible.
  • It must use inexpensive solar thermal panels for gathering energy.
  • The solar panels must be easily produced in an automated fashion.
  • It must have inexpensive (dirt cheap) energy storage.
  • It must produce at least 3 kW of power continuously (24 x 7 x 365 x 20).
  • On a daily basis, it must be capable of gathering two to three times the energy required for a 24-hour period, on the least sunny day of the year. (NREL solar radiation manual)
  • It must be capable of storing the energy required for 3 to 5 days of continuous usage with no energy input.
  • Any person with basic mechanical skills should be able to install the system.
  • The total Levelized Cost of Energy (LCOE) must be under $20 per MWh.

The basic system layout.

Semi-Steampunk Energy Flow Diagram

This system layout image represents the individual pieces and the energy flows between the individual components. The flow controller controls the heat distribution between components.

The system consists of six main components:

  1. Solar thermal cells for gathering energy.
  2. An insulated thermal mass for storing the energy (dirt or water).
  3. A heat radiator for disposing of waste heat.
  4. An LTD Stirling engine for generating energy.
  5. A flow controller for for fluid flow, preventing energy loss from the system, and increasing efficiency.
  6. An inverter to connect to the grid and convert DC power from the generator to AC usable in your house and power grid.

Each component is designed to be as inexpensive, modular, easily replaceable, and mass producible  as possible.

Solar Thermal Panels absorb the sun’s energy in the form of heat. The price for solar thermal panels averages $150 per square meterExtrude plastic cased panels can reduce the cost to $33-$47 USD per square meter, with slightly lower efficiency.

Thermal Mass is a fancy engineering way of saying “insulated pile of dirt or bucket of water.” This is used to store the heat absorbed through the solar panels. The cost of this varies greatly. It can be dirt insulated all around with hay bales and covered with plastic (~$600 USD), four 2,500 gallon water tanks filled with water or sand (~$4,700 USD), a 9 x 20 shipping container insulated and filled with dirt or sand (~$1,100 USD), or an insulated hole in the ground (~$800 USD). This includes the cost of the aluminum tubing which runs from $1.50 to $2.00 per pound. There should be multiple thermal masses, or zones within a single thermal mass, each filled to thermal saturation in sequence.

Flow Controller is used to transfer liquid to and from each of the components. It is designed to keep as much heat in the system, and reuse the remaining heat as often as possible. When the system is energy saturated, or when there is no alternative, it will dump the energy out via the radiator. The multiple thermal masses or zones, at different temperatures, and external temperatures at different times of day, make waste heat reuse an efficient way to extract as much energy from the system as possible. This will run $150 to $300 USD.

Heat Radiator is used to radiate waste heat from the system, or as a heat sink when the system is saturated. This can be a standard aluminum fin radiator and fan, a cold body of water, a hole or trench in the ground with a pipe running into or through it, or any thing else at a lower temperature. The cost varies with type of radiator.

LTD Stirling is the key to this system. The design uses two separate heating and cooling chambers (upper and lower) with a shared piston. The volume is 9 cubic feet (68 gallons). It has 500 sq ft of radiator surface area (floor area of a large two car garage). It is 6.5 feet tall, 3.5 feet wide, and 3 feet deep. It can be vertically or rack mounted. And it is designed to produce up to 6 kW of power, but will be run at 3-4 kW for greater efficiency. The larger these units are, the greater the radiant surface area. The slower they run, the closer they can get to Carnot efficiencies. The full design specs are available here. These units can be daisy-chained together, one to the next. The cost of this device is between $180 and $350 USD.

Grid Synchronized Inverter allows you to attach to the power grid. These are now commodity items and the price for a UL Listed 5 kW unit is from $1,000 to $2,500 depending on manufacturer.

System Cost is based on the location and available kWh/m^2/day (kilowatt hours per meter squared per day) on the least sunny month of the year — for me, that is December. According to the NREL solar radiation manual for where I am, 50 miles south of New York City, that is 1.9 kWh/m^2/day. Over the period of a year, the power varies greatly from 1.9 – 6.2  kWh/m^2/day.

3 kW continuous output, over a 24-hour period, with 30% efficiency, requires we gather 240 kWh to produce the 72 kWh this system will produce over the period of a day. One of our design criteria is, we gather 2 – 3 times the power required for a given day. For safety, the further north you go, the higher the multiple should be. For where I am, it is ~2.5, for Texas 1.9 – 2, for Maine 3.0.

Panel Cost
600 kWh = 2.5 x 240 kWh
315.78 square meter = 600 kWh / 1.9 kWh/m^2/day
$10,428 = 315.78 sq meter * $33 per square meters

Other Costs
$1,100 – Thermal Mass (Shipping container or insulated hole in the ground)
$250 – Flow Controller
$200 – Heat Radiator
$250 – LTD Stirling
$1,500 – Grid Synchronized Inverter

$13,728 — Total Parts Cost

NOTE: None of these calculations take into account the reuse and recycling of the energy gathered, by cycling the energy into other zones or thermal masses at lower temperatures. (IE 90 C –> 60 C –> 30 C –> radiator, where “–>” is the LTD Stirling, and the temperatures are of different zones or thermal masses). Above are worst-case calculations.  

The thermodynamic efficiency of a solar based  LTD Stirling changes based on time of year, and time of day, based on the outside (radiator) temperature. It is not that Carnot takes a holiday (eff not = 1 – tc/th). It is that the temperature differential changes, changing the efficiency. The greater the temperature differential, the greater efficiency of an engine. During the summer months you have a plethora of energy (3x winter), and poor (5%-15%) efficiencies due to a low temperature differential between the heat source and the radiator (outside air). During the winter months you have a high (15%-30%) efficiency, due to the high temperature differential  between the heat source and radiator. This allows the system to generate power in the winter more efficiently, with less energy input. It is counter intuitive and thermodynamics at work. 

Designing the system based on the day-to-day data for Newark, New Jersey over the same time period, taking into account energy reuse and smart energy management, we can reduce our multiple to 2 and only require 288 kWh worth of panels, reducing the panel cost to $5002.10 USD and the system cost to $8302 USD. With economies of scale and alternate production techniques, increasing the thermal efficiency of the panels (1), further cost reductions are possible, reducing the system cost another ~$3,000 USD, making the system cost approximately $5500 USD. The cost would be lower in southern states like FL, TX, AZ, southern CA. 

Total Energy Output over the period of a day is 72 kWh of energy. With a lifespan a 25 years, the total power output is…

657,000 kWh = 25 year * 365 days * 24 hours * 3 kWh

657 MWh total energy produced over the lifespan of the Stirling.

Levelized Cost of Energy or LCOE is basically the the cost of the generating plant, fuel, and maintenance over its life span — minus subsidies — divided by the total energy generated over the period of a generators life span.

The LCOE for the first non-optimized design is $13,728 / 657 MWh or $20.89 USD per MWh. Optimizing just a little brings the LCOE to $8,302 / 657 MWh or $12.66 USD per MWh. Allowing for economies of scale, automation, home building techniques, reduced energy costs in manufacture, and other things this article didn’t have room for, gets the LCOE to $5,500 / 657 MWh or $8.37 USD per MWh.


Comparing the current cost of energy at ~$100 USD per MWh to a system based on a redesign of a 100 – 200 year old technology shows that sub $20 USD per MWh energy is possible with technology available today. It also shows that renewable energy can be far cheaper than fossil fuels with a little creativity.

David Fuchs is a classically trained engineer and programmer. He is involved in open sourcing, software, and hardware. Current interests: 3d printing and nanotechnology, predicting the future of technology, and low-cost power production in developing nations using material at hand. You can check out his website for more of his writing, and you can contact David on Google +

Tags: , , , , , ,

About the Author

is many, many people. We publish a number of guest posts from experts in a large variety of fields. This is our contributor account for those special people. :D

  • Jack Panella

    What kind of mcmansion are you running that you need 3kW continuously?

    The sky is always colder. See US Patent 4624113, and then dig into the math of radiative cooling (Berdahl-Martin, Smith, etc.) It might not give you 3kW/513m^-2, but you won’t need the 513m^-2 of solar panels and humongous thermal mass, and it will be much more available than solar.

    Replace your refrigeration and air conditioning with the same tech, and you don’t need that much electricity. In Jersey, heating is a big power sink, but there must be a better solution than generating power at 30% efficiency and then taking another efficiency hit to produce heat, which was the original energy source.

    If you’re running a machine shop, I don’t know what to tell you.

  • Technology Submission – Novel Rotary-Turbo-InFlow Tech – Featured Development

    Atypical InFlow Thermodynamic
    Technology Proposal Submission
    Novel Fueled Motor Engine Atypical Type

    *State of the art Innovative concept Top system Higher efficient percent.
    Have similar system of the Aeolipile Heron Steam device from Alexandria 10-70 AD. -New Form-Function Motor-Engine Device. Next Step, Epic Design Change, Broken-Seal Revelation. -Desirable Power-Plant Innovation.

    YouTube; * Atypical New • GEARTURBINE / Retrodynamic = DextroRPM VS LevoInFlow + Ying Yang Thrust Way Type – Non Waste Looses

    -This innovative concept consists of hull and core where are held all 8 bteps of the work-flow which make the concept functional. The core has several gears and turbines which are responsible for these 8 steps (5 of them are dedicated to the turbo stages). The first step is fuel compression, followed by 2 cold turbo levels. The fourth step is where the fuel starts burning – combustion stage, which creates thrust for the next, 5th step – thrust step, which provides power to the planetary gears and turbines and moves the system. This step is followed by two hot turbo steps and the circle is enclosed by the final 8th step – bigger turbine. All this motion in a retrodynamic circumstance effect, wich is plus higher RPM speed by self motion. The Reaction at front of the action.

    *8-X/Y Thermodynamic CYCLE – Way Steps:
    1)1-Compression / bigger
    2)2-Turbo 1 cold
    3)2-Turbo 2 cold
    4)2-Combustion – circular motion flames / opposites
    5)2-Thrust – single turbo & planetary gears / ying yang
    6)2-Turbo 2 hot
    7)2-Turbo 1 hot
    8)1-Turbine / bigger

    -With Retrodynamic Dextrogiro vs Levogiro Phenomenon Effect. / Rotor-RPM VS InFlow / front to front; “Collision-Interaction Type” – inflow vs blades-gear-move. Technical unique dynamic innovative motion mode. [Retrodynamic Reaction = When the inflow have more velocity the rotor have more RPM Acceleration, with high (XY Position) Momentum] Which the internal flow (and rotor) duplicate its speed, when activated being in a rotor (and inflow) with [inverse] opposite Turns. The Reaction at front of the action. A very strong Novel torque power concept.

    -Non waste parasitic looses for; friction, cooling, lubrication & combustion.

    -Shape-Mass + Rotary-Motion = Inertia-Dynamic / Form-Function Wide [Flat] Cylindrical shape + positive dynamic rotary mass = continue Inertia positive tendency motion. Kinetic Rotating Mass.

    -Combustion 2Two continue circular [Rockets] flames. [ying yang] opposite one to the other. – With 2TWO very long distance INFLOW [inside propulsion] CONDUITS. -4 TURBOS Rotary Total Thrust-Power Regeneration Power System. -Mechanical direct 2two [Small] Planetary Gears at polar position. -Like the Ying Yang Symbol/Concept. -Wide out the Rotor circumference were have much more lever [HIGH Torque] POWER THRUST. -No blade erosion by sand & very low heat target signature profile. -3 points of power thrust; 1-flow way, 2-gear, 3-turbine. *Patent; Dic. 1991 IMPI Mexico #197187 All Rights Reserved. Carlos Barrera.

  • ChristianHJW

    You should have a look at this :

    If this works, no Stirling will be needed anymore …..

    Best Regards from Germany

    C. Wiesner

  • DracMorair

    David, future is looking brighter! Pun intended. Independence Energy Alliance. Sometimes, and there’s an old movie to attest to it, going from concept to application gets lost. Thought ya might want to take a look at the people taking ideas like yours and offering them to John Q Public.

    • David Fuchs

      What I am aiming for is individual energy independence. Not power fed from the grid. You see that is the future. When energy drops below about $20 per MWh the energy grid is no longer sustainable without massive subsidies This will happen in the next 10 years. I just hope to get there first. 🙂

      • Bob_Wallace

        “When energy drops below about $20 per MWh the energy grid is no longer sustainable without massive subsidies ”


      • DracMorair

        I have to agree with Bob here. The private sector (such as the IEA) can handle this very real issue to private entities. Using the infrastructure already in place to provide clean energy to the masses without regulatory capture, government buerocracy and the overhead. Now I might have minored in economics but even I can see getting into the 20’s -30’s by that alone. Agorism at its finest.

  • Ann E Mouse

    The creation of a viable $200-$400 LTD Stirling _alone_ would cause boom in home grown energy systems – I’d use a concentrator and just run it during sunny times.

    More expensive silicon steel may be desirable for some of the magnetic bearing components or UHMWPE for some of the critical wear components could provide reasonable performance but require periodic field replacement.

    cheap mass produced Stirlings are a holy grail to transform the small engine/energy world – small boat engines, industrial motors, ag processing of grain, etc

    • David Fuchs


      These are not small engines. For 3 kWh you are talking 6 x 3 x 3 foot (2 x 1 x 1 meter). Using it on a small boat would be fun to do, but I do not think it would be practical from a floor space required perspective. It would be an interesting design challenge. Use an oversize rudder that doubles as your heat sink, use a small continuous feed (no tank) gas powered hot water heater as the heat source.

      Powering Industrial motors, ag processing of grain, etc is all workable though.

      A couple years back I built an x – y magnetically locked slide (free to move in the z). It was able to support about 200 lbs with a little less than a mm of give in the y. Now take a 5 to 10 pound, 12- 16 inch diameter piston, mount and center it in the cylinder using one of these slides, and once the initial wear (1/8 to 2/3 mm) occurs, that is it, no more wear. Well, as long as the curie temp on the magnets isn’t reached that is. So as long as the magnets last the piston doesn’t have to be replaced.

      Ann this is for the next commenter … who I know will happen soon. 🙂

      Now I have been told I need to explain things better, and since I know the very next comment is going to be ….

      But, but, (insert random negativity here)

      But, but, with a 16 inch diameter piston and a 2/3 mm – .667 mm gap (0.0262598 inch) and a circumference of 50.265 inches, an area of 201.062 in2, that would be like having a 1.318 in2 inch hole in the piston.

      A 1.32 in2 hole is different than a narrow gap of the same area due to boundary layer effects (resistance) that occurs in narrow channels. Plus add in the fact that you are pumping a hundred plus cubit feet of air into and out of the piston(s) every minute and the leakage around the piston is minimal. Plus that 2/3 mm gap is the high side of the possible wear.

      Now I fully expect the next person to quote … “hundred plus cubit feet of air” … and I look forward to it.


    • David Fuchs

      Now I have been told I need to explain things better, and since I know the very next comment is going to be ….

      But, but, (insert random negativity here)

      But, but, with a 16 inch diameter piston and a 2/3 mm – .667 mm gap (0.0262598 inch) and a circumference of 50.265 inches, an area of 201.062 in2, that would be like having a 1.318 in2 inch hole in the piston.

      A 1.32 in2 hole is different than a narrow gap of the same area due to boundary layer effects (resistance) that occurs in narrow channels. Plus add in the fact that you are pumping a hundred plus cubit feet of air into and out of the piston(s) every minute and the leakage around the piston is minimal. Also, that 2/3 mm gap is the high side of the possible wear.

      Now I fully expect the next person to quote … “hundred plus cubit feet of air” … and I look forward to it.

  • Guest

    Hi David, thank you for posting this very intriguing idea. Two questions: the solar panel Price you assume is as you say 1/4 of current prices. How did you calculate the production cost of your panel design, and would the system still be Price competetive with e.g. natural gas, if we in a conservative estimate were to only assume existing panel prices?

    As mentioned in a post below, good commercialization would be key to large scale implementation. Rooftop solar PV soft Costs represent 2/3 of total LCOE, and if we assume the same for your system (and current panel prices) will it still be price competetive?

    • David Fuchs

      The cost of the panel factory is roughly 3-5 million to build. Extruder, injection molding, sonic or heat welders for plastic, collector panel forming and welding, final assembly mechanism. The factory cost is inconsequential due to depreciation and the tax laws. Human labor cost are minimal due to the automation and the panels only having only 6-8 pieces designed for automated assembly. Most of the automation resembles 1950’s cannery tech.

      Basically, the cost comes down to land and taxes, raw materials, minimal human labor, and shipping costs.

      “only assume existing panel prices”. Ahhhh, no I will not assume that. Solar thermal panels price have not seen the sort of downward price pressure that photovoltaic panels have. It is time to change that.

      The second part is another question containing the assumption “(and current panel prices)”. So grab a calculator, and the rest is left as an exercise for the reader.

  • A couple points:
    Can I buy a 6 KW heat engine/generator for $350? I’m skeptical. And can I build simple heat collectors myself for less than $10,000?

    But I don’t really need a 6 KW generator. I need maybe 1 KW to run my house (I’m a cheapskate) as a base load. Also, your panel size is about ten times the usable area of my roof.

    In the winter I’ll be dumping the “waste” heat into my house air. How much is that worth? And in the summer I’ll dump it into my 1000 gallon dug well that’s always 55 degrees (english units, sorry).

    Microinverters are the rage in PV. If you could build a single panel storage/generator/inverter, you’d have a reasonably priced, scalable, single unit that could be unpacked, mounted, and plugged in the wall. With the money you saved, you could buy another soon. Or put one in an odd place.

    • David Fuchs

      It is actually 3 kW. If you run it at 6 kW, the efficiency tanks through the floor. The data provided for the panels is from the NREL manual (see link in article) they used the same panel types from 1961-1990 for their data collection. They are obsolete, old school and inefficient. Hence square meters of panels required based on the NREL numbers. There are better solar thermal collectors available now. It is overkill.

      The criteria are massive overkill. Criteria of 5 days energy storage without sunlight, 2-3 times the energy needed on the worst day of the year, worst solar thermal panels in existence, running at a low temp differential, etc.

      If you are planning on surviving a nuclear winter, some random god blotting out the sun for a week, or a zombie apocalypse build the above system. It is total overkill.


  • Dale Kaup

    Why not use waste plastic soda bottles filled with water as thermal mass. Alternately a cheap brand of soda or water in aluminum cans filling a shipping container would be great.

    It could also serve as a source of safe drinking liquids. At a certain point the water become more important than the electricity after all.

    • David Fuchs

      Problem with that idea, you would need 40,000 one liter bottles all daisy chained together with hoses. That is 40,002 points of hose failure and 40,000 points of bottle failure to worry about.

      All kidding aside, the final design is being reworked for an 11th time. I am working the storage temperature (reads efficiencies) up and the size down to reduce cost even further.

      The target is the thermal mass at 150 C +-10 C. So water is no longer an option as it would boil. While it would be fun to watch this thing reach 150 C full of soda cans, sand or dirt is the better option now.

      You did give me a really good idea. At 150 C the thermal storage unit can be outfitted for cooking. That would alleviate a ton of issues in the world, heat to sterilize water, fuel for cooking, etc. The great thing is it would all be waste heat from leakage.

  • I suggest using straw rather than hay.

    • David Fuchs

      Someone else suggested that. Is there a difference and what is you reasoning?

      • Chris Burke

        Not sure what John’s reasoning is but hay is for feed and straw is for bedding. Hay is far more nutrient dense and about twice as expensive. Straw is the stalk of the grass and basically just what the name implies, an empty tube.

        • Bob_Wallace

          In straw bale building the rule of thumb is to use straw. Hay can contain seed heads which furnish food for rodents. If the protective shell is damaged you can get families of mice/whatever eating up your insulation.

          Plus, as Chris says, straw is cheaper.

          Rice straw is particularly attractive for insulation. It has a high silica content and is very rot resistant compared to wheat or oat straw.

          Straw bale construction is pretty well developed. Take a look at how folks are building super efficient buildings for modest costs and you should get some good ideas for long term, highly insulating enclosures. There are straw bale houses in Nebraska that were built in the 1930s.

          • David Fuchs

            I helped build a hay-straw bale house in Arizona. It is what gave me the idea to use them as insulation.

        • David Fuchs

          Thanks. 🙂

  • Dipesh

    hi David,

    Thank you for the article,. Im from Nepal where there is a power cut for 8 to 17 hour a day for 365 days, which has made our life difficult , Im an Architect always looking for smart solution to address this kind of issue either by implementing directly to my project for influencing client to implement.

    If the product you have elaborated work as you have said, I will be happy to try, if it can be easily use on house hold level. As I believe to impact on big mass you need to impact on small one.

    And I believe that this can be easily implement in the county like us were if you promise to provide electricity whole day long .

    Hope to get regular updates on this

    • Bob_Wallace

      Dipesh – just look to your neighbors, India and Bangladesh. They have figured out how to install millions of solar systems at very affordable costs for those without electricity. People can purchase their own solar system including LED lights and storage for less per month than they now spend for kerosene.

      Installing a lot of small scale distributed solar would take a lot of load off your grid and let you use your hydro in a more effective manner. And it would mean that people would have light 24/365 without having to resort to candles and kero.

      Install a lot of solar. You’ve got excellent hydro to fill in around the Sun. Small amounts of battery storage would carry households through the night and the grid could be shut down when people are asleep, saving that water for when people are awake.

      No need to wait for something new to be invented. If something better comes along, fine, switch to it. But in the meantime there is an answer right now.

      • Dipesh

        Hi David, – I totally agree with you and also gone through the model that has been used at our Neighbor country to distribute Solar power. But just to give you an Idea regarding the cost per watt, It will cost around $4.5 to $9 per watt ( depending upon the size of solar panel and its system), which is too expensive for the people of country whose per capita income is $742 . Some financial institute do finance on solar but it is not accessible to people who needed the most. To make all people accessible to that fund, we (team) were working on the model or scheme which will allow the people to install solar power on there roof.

        May in soon I will post that scheme on this forum so that you can give us yous valuable suggestion.

        Note: My company where I work at the moment totally runs on Solar power.

        • David Fuchs


          From what you have said you need a really inexpensive power source. Give me a price range, a power output, and voltage-cycle range, I will see If I can hit the target on all three.

          The system I thought was finished, and still seem to be redesigning, was meant to compete with western energy prices. It seems to need a little brother.

          David Fuchs

    • David Fuchs

      Digging through a bunch of doctoral dissertations, and masters thesis. I think I might have found an interesting solution no one has seen before.

      I have been designing a system from the perspective of the western world. What if I change the thought process and combine clean water – cooking, heat, and electricity in one unit at really low cost.

      By combining Compound Parabolic Collectors, a drum to store the heat, and an LTD Stirling built into the drum, it is possible to do heat electricity and cooking in one unit for a small house.

  • @David Fuchs: What is the input temperature requirement of the Stirling engine in your design?

    Solar collectors are really efficient only if their output is not much higher than 45C. In summer, this can go up to 50-60C but in winter, 35-45C is the targeted top temperature and they will never go above 50C (at least my relatively modern, 12KW evacuated tube system here in Hungary at 0C outside temperature).

    An other LTD Stirling design requires input heat at 180C which is ridiculous since the collectors will never reach this work temperature without complex/expensive concentrating components.

    • David Fuchs

      Are you are talking water temperature in the tank or panel-tube temperature?

      • I am asking about the temperature of the water that goes to the “hot” side of the Stirling engine.

        The maximum water temperature in the storage tank is the same as the maximum temperature achieved by the solar panels (usually even lower since heat dissipates in the tank to a certain extent).

        • David Fuchs

          Evacuated tube thermal can run up to 200 C (from memory), depending on tube length-width, vacuum level, type of glass, etc. I think your system is regulated down to the 55-60 C range for safety ie to prevent boiling and tank explosions.

          The temperature going into the Stirling depends on the time of year.In the summer it will be in the 90+ C range, In the winter around 80+ C.

          If I decide to run mineral oil instead the temperature will be higher.

          • Theoretically, the evacuated tube system may be capable of handling the 200C but efficiency would go down severely. Using oil as the transfer fluid wouldn’t eliminate this problem.

            Solar collector efficiency is very dependent on the heat difference between the heat transfer fluid (water or glycol) and the outside environment (dT) – especially for the flat-plate variety. The higher efficiencies (60-70%) are achieved only when dT is relatively small (say 10C, 20C for flat-plate, more with evacuated). When dT is higher, efficiency is going down. This is why solar controllers always heat the tank up gradually and try to keep dT down as much as possible.

            Flat-plate collectors would be completely unsuitable for such high temperatures but even evacuated tube collectors would loose a lot of efficiencies.

            I believe that those (80C, 90C) temperatures are too high for the collectors and the resulting system efficiency would be too low (at least in continental climates).

            If your Stirling engine could work with lower temperature input water on the hot side, the resulting system efficiencies would be high enough to warrant the added system complexity over a simple hot-water system or a PV system.

          • David Fuchs

            “If your Stirling engine could work with lower temperature input water on the hot side, the resulting system efficiencies would be high enough to warrant the added system complexity over a simple hot-water system or a PV system.”

            The problem with that is the max efficiency in a heat engine is 1 – tc/th, so lower temperatures are not an option. Due to losses and inefficiencies, you need to gather more energy at lower temperature differentials, which means more panels and higher cost.

  • David Fuchs

    I was IM’d tonight and asked a question. I thought I would answer the question here.

    “How much energy is stored in a shipping container when it gets 20 degrees C hotter.”

    In a 20 foot by 9 by 9 foot shipping container you have about 33 cubic meters.
    Lets use sand since it is cheap and easier to store than water. Sand has roughly half the thermal storage capacity of water.

    Heat stored in 33 m3 of sand heated 20 C can be calculated as

    q = (33 m3) (1800 kg/m3) (835 J/kgC) ((90 C) – (70 C))
    = 991,980 kJ
    = (991,980 kJ)/(3600 s/h)
    = 275.55 KwH

    So for a 20 degree C rise in temperature you have stored a quarter of a megawatt hour of energy. To put that in perspective. That is roughly my car on a race track for an hour or a mid sized house for a week.

    see also –

    • Asbjørn Søndergaard

      Hi David, thank you for an intriguing post.

      If I summarize your numbers correctly and they can be validated by a physical prototype, what you are proposing can basically spark a substantial gamechange in global energy production because the proposed system solves all current challenges in renewable energy in one go: price-efficency, intermittancy, availability, producability.

      I’m not a trained engineer, so I’m not capable of fully assessing the solidity of your calculations. But as a dedicated pro renewable energy enthusiast, I warmly welcome this contribution. If I may, I’d like to ask here in the forum some questions on the next steps, as others might have same curiosity:

      Are you planning to commercialize the results? The reason I’m asking is, that in case this idea should become a reality and go from an open-hardware initiative to something that will significantly impact energy-production, it needs a commercial manufacturer that will provide total installation solution for the end customer, take care of continuous system design optimization, and provide maintainance services once installed.

      This might seem like a banality, but never the less there’s a multitude of bright ideas in energy production that struggle on entering the market on this exact point.

      If you are not planning to commercialize the idea, will you then seek to provide more solid evidence of its viability? The first question anyone attempting in some form another to commercialize such a design would meet, would be the too-good-to-be-true skepticism. A peer-reviewed paper in an internationally credited engineering journal with the system design and calculations could be an important first step; the demonstration of a working physical prototype with continous output validated by a acknowledged laboratory another.

      In any case, best wishes to the progress of your idea!

      • David Fuchs

        ” the proposed system solves all current challenges in renewable energy in one go: price-efficency, intermittancy, availability, producability.”

        Those were the goals of this design.

        “I’m not a trained engineer, so I’m not capable of fully assessing the solidity of your calculations. ”

        The calculations are based on an computer evolved design for a displacer-regenerator-heat recovery system. Until it has actually been test and not just simulated, the numbers used for efficiency should be around 10%. Which still gives us sub twenty dollar a MWh in the southern US and mid twenties is the northern states. That is with double pane or honeycomb (TIGI inc) style solar panels.

        I am going to be doing a an article on the panel design for CleanTechnica in the next month. I have no idea how I can take such a boring subject and turn it into something fun to read.

  • question

    Very cool. This sounds like a perfect project for crowdsourcing. Try Kickstarter or a similar site do do a prototype. I suspect you’d get a lot of buy in. If you can make it work in reality versus theory you’d be a long way to having something very important.

    • David Fuchs

      i am going to bounce the LCOE calculation off the people at TED.COM and if I am correct in my numbers, I am going to ask about funding options as my next question there. Crowdfunding is one of the options I was planning on asking about.

Back to Top ↑