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Split Solar Stove
  • There are many solar stove designs. They either don't concentrate sun's energy or if they do, don't control it. Above all, they require the user to be outdoors while using - which is a big hurdle for adoption.

    Here is a design which 'splits' the system into two - viz., an outdoor solar concentrator and an indoor stove. The heat focussed from the outdoor concentrator is to be transferred indoors using a heat pump. By splitting the stove, we also get to 'scale' the outside to make it work as effectively as other denser forms of energy. We will also have to use an onboard microcomputer to control the outdoor unit (ON/OFF, swivel the outdoor unit to track the sun (if a lens based concentrator is deemed suitable), control intensity of heat generation and ability to 'simmer' down by diverting the heating fluid in the heat pump to a cooler)

    By not converting energy from one form to another, we avoid conversion losses.

    As I see it, the basic scientific principles being employed here by themselves are OK. However, I do understand there are theoretical limits to everything, especially energy. I'm of the opinion that if this statement is true, then this project is mainly a challenge of high tech engineering (in designing the
    physical form to be effective in transferring heat, in "teaching" the art of control to the microcomputer onboard, in choosing the right materials (possibly even materials that are only available in the fossil-fuel framework, such as certain cooking-friendly hydrocarbon-based heating gases :( )). The microcomputer by itself could be tiny enough to be powered using minimal solar PV or using some other forms of energy, such as thermocouples.

    I see the dependency on fossil fuel infrastructure as an inherent feature of our lives today. We must wean off it, but to do so, we must get started... and time is running out.

    Is this feasible? If not, why not? How and where do we get started?
    split-solar-stove-overview.png 43K
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  • My first thought is that your collector will have to be hotter than your burner. For your burner to be hot enough to cook food the tube conducting the heat in from the collector is going to be hot enough to burn wood. Basically, I'm picturing a red-hot wire passing through the wall of your house.

    It might be more straight forward to just slide the cooking area through a hole in the wall (convert a window?) so that it rests under the collector, then roll it back inside when it's done cooking.

    You could use an array of mirrors to heat the collector past any reasonable cooking temperature. I've seen DIY versions where a 1mx1m array was used to melt lead shot. Also, you could probably just make the mirror array manually controlled. The operator would just have to come back and adjust it every hour or two. Maybe run some cables into the house so they don't have to go outside. 
  • Thanks Matt.

    Please correct me if I'm wrong: To make this work as fast as a fossil
    fuel source (like LPG), we will naturally have to scale the outdoor
    unit, but that doesn't mean a single large outdoor unit but instead, it may be useful to imagine a series/parallel array of several concentrators. Yes, it will be
    expensive but it could be meeting some kinds of users (like
    restaurants or the very rich, who may turn out incentivizing such a business. I also have a hybrid-oven-stove design in mind, but if it must reach the common people, we must 'extract capital' from the gradient :) ).

    The other problem with 'cooking' itself is that stuff like "pan cakes"
    can't be made through a "push into a hole-in-the-wall-oven" since it
    requires active attention by the user. In India, our staple food being "Roti" and
    "Dosa" makes such an oven design even less sale worthy (although,
    arguably the oven model will be highly energy efficient and I love the idea).

    How does one go about 'trying' this out without actually wasting materials/money? Is there a open source/free software for doing thermal engineering designs and simulations? Do I necessarily have to know calculus and numerical methods? (I don't, though I can program computers and may manage to pull it off if there is a simplified way of doing 'calculus'). I'd like to know how do we start figuring out the 'sizing' and the materials to be used? (What gas, how big will be the heat transfer pipes, etc,.)

  • I don't think you've developed the idea to a stage where you would benefit from any kind of simulation or prototyping. I suggest you clarify exactly what the goal is and how you propose to achieve it.

    A stove/oven will always get hot enough to be dangerous. If you're piping in that heat from outside then the heat collector has to be even hotter, which means even more dangerous. More importantly, the thing conducting the heat has to be incredibly well isolated from all the flammable stuff around it. That means thinking along the lines of a thick ceramic/masonry shell (the equivalent of a chimney). 

    As for the heat transportation mechanism, you might consider a heat pipe with something a bit exotic like cesium or potassium in it.

    You could probably put together a spreadsheet with some basic equations in it. Look into the math for solar thermal concentrators. It's relatively simple to figure out how much solar energy falls on a certain spot and how much of it gets reflected by a mirror and how hot that would make the thing it's all concentrated on. 

    Something you might consider is that the majority of the system you're proposing would be a regular solar thermal collection system, which is normally used to heat up water (instead of a furnace). Since the amount of time a person would spend cooking would be short compared to the amount of daylight, it makes more economic sense for them to store that heat in their hot-water tank as a default. If they want to cook, they can just flip a switch and point all the mirrors at their oven thing, then when they're done cooking they can point the mirrors at the hot water collector again.
  • The idea is good. You have to find a liquid that can be heated to 200 degree celsius without evaporating that can transfer heat to the 'roti' or 'dosa'. I guess cooking rice or making steamed idlis would take a lot lower temperature. You can probably control this temperature by regulating the flow of the transfer fluid. It is worth a try. A prototype solar cooker just for experimental purposes should not cost a lot of money. You could even try cooking oil as your heat transfer fluid. I believe if you could bring the price down to around two thousand rupees, you really have a fantastic chance of creating a successful commercial product since the LPG prices are rising rapidly in India.
  • I guess you guys haven't come across the Auroville solar cooker, which does just this: . It uses stationary spherical mirror that makes 150°C steam which is piped inside to the cooking surfaces and cooks 1000 meals a day. I don't know if it has a steam accumulator but with a lighter load and a vacuum insulated collector I'm sure it could get to over 250°C - plenty for cooking. A steam accumulator is a heat energy storage device that condenses steam under pressure and then when the pressure drops it re-evaporates (used in power stations to cope with load fluctuations). This steam could heat a cooking surface enough to fry, and would work at night or even for a few days of cloudyness if properly insulated. All pipework would have to be insulated too, of course, and if the collector is above the store then it will need a pump that can deal with the heat and pressure (36bar @ 250°C). One day I will get around to building this :bz but I'm up to my eyebrows in electronics and architecture at the moment.

    I have also designed the cooking surface control. My method is to have an on-off switch (steam valve) and a separate (although could be integrated into the same knob, like lights dimmers have a switch that clicks as well) heat flow control which works by varying the amount of conductive contact between the steam heat exchanger and the cooking surface.

    Once you have high grade heat on tap you can then start thinking about other heat users such as converting a gas fridge to run from direct heat (they need ~205°C to run) or a sterling generator (more efficient than a steam engine which loses all of the latent heat of evaporation in the cycle). The inefficiencies of the sterling generator at low temperatures generate quantities of low grade heat that can heat your hot water and the ground beneath your house for home heating too, for a complete solar home energy solution. Storing cool from a large heat powered fridge system (they have no moving parts so they don't wear out) could allow reliable solar freezing and air-cooling too if needed.

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