These are great questions to make me consider whether this idea is practical. I really appreciate your help in that regard.
S Bengi wrote:How much heat and electricity do you need per day for your trailer?
If I only generate electricity, in kWh/day I need a minimum of 1.8, maximum of 13. Most days we would use around 5, exactly like your estimate.
S Bengi wrote:Do you have enough roof to collect all of this heat and electric?
No but I plan to build wings so the panels will lie flat against the sides of the trailer while driving, then raise up above the trailer while parked. This will be large enough to hold the 21 240W (totaling 5kW instantaneous) panels I bought from SanTan Solar for $735. I have an MPP Solar Hybrid LVX 6048 I bought from Watts247 for $1,577, and if I were to build the 15-16 kWh battery bank with LiFePO4 batteries I've been planning, the cells would cost around $2,483. The total for these items (not counting wires, racking, 48-12V DC buck converter, etc.) is $4,795.
Battery cells:
https://www.mobile-solarpower.com/raw-lifepo4-deals-page.html
Battery designs:
https://www.mobile-solarpower.com/design-your-own-12v-lifepo4-system.html
S Bengi wrote:Current Tech for a 33ft long trailer:
5kW or 15kWhr/day Solar PV Array @ $5,500.
10kW or 30kWHr/day Solar Thermal @ $5,500. It sits under the regular solar panel, about 30F above ambient temperature
Thank you very much for telling me about sundrumsolar.com! I've been looking for a suitable plate heat exchanger, and theirs looks to be exactly what I want. I like their general idea of using a heat pump to collect heat even on cooler days.
S Bengi wrote:1kW or 6kWHr/day Heat Pump COP=5 @ $1,000
7.5kW Hydrid Inverter/Charger @ $5,500.
7.5kW or 15kWHr LiFePO4 battery @ $5,500.
Proposed Tech for a 33ft long trailer
Given that it cost about $11,000 for a 7.5kW, 15kWHr/day battery+inverter/charger setup.
I think this estimate assumes I would be buying new panels, off-the-shelf batteries, and perhaps pay a professional for the design & install. Since I'm building the PV array & battery, I expect to have a lower cost than this estimate.
S Bengi wrote:And it takes up about 6ft3.
How much would your sand battery+rankine turbine system cost and how much space would it take up?
I don't know yet. My thought is to fit it onto the tongue of the trailer, where the propane bottles are now, and wrap it with a fairing.
S Bengi wrote:To get 15kWHr out we would have to store 45kWHr due to losses
If this is the case my idea is not practical, because LiFePO4 batteries have a 90-95% round trip efficiency.
S Bengi wrote:And to store 45kWHr we would have to put in 50kWHr
Because we need to maintain a temperature delta, lets keep the state of discharge at 50% so really we need 100kWhr of storage
High grade heat (650C+) in the sand battery, and high grade cold (-196C) in the liquid air battery would not need to be kept below 50% state of discharge because I think the cryophorous cold steam Rankine cycle can run from 100C all the way down to close to 0C.
S Bengi wrote:How much space will the 100kWHr thermal sand battery take up at 485F?
Glass melts at 1400-1600C, so I'm estimating a sand battery could be as hot as 1200C in a glass container, and be cooled as low as 0C by the cryophorous system. With that temperature range, 16kWh of heat could be stored in 57 kg of sand, which is about 85 liters, or 22 gallons, and 100 kWh could be stored in 353 kg, which is 530 liters or 140 gallons. But if the sand battery's max temperature was 485F, the amounts would be 1,688 kg, and 2,532 liters or 669 gallons, which is impractical for a trailer.
S Bengi wrote:How will you get the energy in the sand battery (wires or tubes)? What will you use for the energy source: regular solar electric or will it be solar thermal?
I'm not sure yet. Maybe a heat pump which can transform low grade heat into high grade heat (1200C), and use steam as the working fluid, so that means tubes. Electrical resistance heating elements can get that hot easily; I don't know whether there are heat pumps which can do so.
So I'm thinking the main heat source for the sand battery would be solar thermal run through a heat pump, with solar electric as a secondary source.
S Bengi wrote:Or will it be a fancy heat pump with a temperature delta of over 400F what will the COP be 1?
The COP of a stirling engine is about 0.4 (40% efficiency), but it can have a very high temperature delta (-196C to 700C+; a delta of 1,613 degrees F), which you can see at
https://www.youtube.com/watch?v=GFfMruoRMGo.
S Bengi wrote:Can you send a link to a 7.5kW rankine turbine that operates at 485F to 235F, I would love to see the price and specs and size
I don't have that information. Charlie Solis might be able to answer it; see the following:
4+ kW turbine, which could be run at combustion temperatures - https://www.youtube.com/watch?v=nndCJixLhiE10 inch turbine stats - https://www.testurenergy.com/10-inch-turbine-system$20,000 for complete 10kW solar thermal collector, 50kWh storage, 10 kWh electrical / 20 kWh thermal generation system - https://www.testurenergy.com/solar-cryophorus-turbine-system