Sand battery + liquid air

I've been thinking of implementing an alternative energy system for my camping trailer, which I'll summarize very briefly.

1.  Collect electricity & heat from solar panels.
2.  Store some electricity in lithium iron phosphate batteries for immediate power needs.
3.  Use a Tesla-inspired cryophorous vacuum cold steam Rankine cycle thermodynamic transformer (a Tesla turbine at the center) to separate heat into a hot water tank and cold into a cold water tank, and run it the opposite direction to generate electricity when needed.  See Jeremiah Ferwerda's and Charlie Solis's Youtube channels for more info on this.
4.  Use similar heat pumps to store high grade heat in a sand battery and high grade cold in a liquid air battery, and use the heat & cold to generate electricity in the thermodynamic transformer (step 3).
5.  Generate drinking water from the cold side by condensing it with more efficiency using MOF-303.
6.  Use the liquid air to cool the refrigerator, freezer, and directly to provide air conditioning.
7.  Use the heat to provide space heating and domestic hot water, and directly heat the oven and stove (pipe through the stove's burners with no flame or resulting indoor pollutants).

One key problem I'm working on right now is that the cryogenic compression & cooling steps are inefficient, so the round trip efficiency of a liquid air battery is about 50%, which is far less than the 90-95% round trip efficiency of Lithium iron phosphate batteries.  I think some of this inefficiency can be offset by using the stored heat and cold directly as described above, so avoiding converting electricity to cold air in the air conditioner, freezer, and refrigerator, and avoiding the need for trips to buy propane and fill up the fresh water tanks.  But I'm not yet convinced those offsets are enough to make this a good system.

Another key problem is that I'm not sure a Tesla turbine can work as a heat pump (in the storage cycle), and while Jeremiah Ferwerda and Charlie Solis indicate the Tesla turbine can be 90% efficient (in the electrical generation cycle), many studies have found an efficiency closer to 40-50%.

I'm interested in any feedback on these ideas.  This is only a rough outline; I have documented much more of the details but have not decided this is actually a good idea yet.

Tim

Cool stuff! But a lot of moving parts. Offhand, I wonder if there's a minimum scale at which this becomes viable?

I'm making popcorn and watching this thread!

Douglas Alpenstock wrote:Cool stuff! But a lot of moving parts.

Yes, and the many parts could break due to the trailer's vibration on the road. If these ideas were used for a house, vibration would be less of an issue.  The turbine's noise could be a problem too, although noise is a function of mechanical balance and efficiency.  I think though that the moving parts would literally be valves, switches, sensors, pumps, and turbine bearings, all of which could be selected to handle vibration and generally be available as off the shelf components which would be relatively easy to replace.

It would definitely be simpler and more solid state to just use a lithium iron phosphate battery.

How much heat and electricity do you need per day for your trailer?
Do you have enough roof to collect all of this heat and electric?

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
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. And it takes up about 6ft3.
How much would your sand battery+rankine turbine system cost and how much space would it take up?
To get 15kWHr out we would have to store 45kWHr due to losses
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
How much space will the 100kWHr thermal sand battery take up at 485F?
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?
Or will it be a fancy heat pump with a temperature delta of over 400F what will the COP be 1?
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

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=nndCJixLhiE

10 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

Overall it sounds like an expensive, inefficient, and brittle system with many points of failure. There are a lot of moving parts to wear out, and in a moving vehicle this is going to be accelerated. And when failure does occur there are some rather scary failure modes (liquid air spill?)

What I'm not sure about is why you are exploring this idea? What problem does this solve for you that an off-the-shelf battery/inverter system does not?

I had a conversation a few months ago with person who works for a company that are using broadly similar ideas for grid scale energy storage - liquid air, stored separately from the heat. When energy demand on the grid spikes they pass the liquid air through a heat exchanger where it rapidly flashes to vapour and drives a turbine. They have had teams of engineers working on this for a couple of years and are still 12 months from a testable grid scale demonstration. She estimated that they - with a very careful design focusing on efficiency - will get 70 to 80% efficiency of energy stored --> energy returned to the grid.

Design issues they were dealing with were bulky storage tanks, and the thickness of insulation needed for efficiency. Storage alone, given the bulk, seems like it would be tricky for a vehicle?

Sounds like you already know what logic and math are telling you. A cheaper more efficient off the shelf solution is definitely the battery, solar, air to air heat pump route. You could incorporate mass into the trailer easily to act as a battery extender but Insulating the trailer better would pay much better dividends. I find it is a common theme in these threads; we all want to solve these problems on the supply of side when it's much cheaper to solve on the demand side...

Michael Cox wrote:Overall it sounds like an expensive, inefficient, and brittle system with many points of failure. There are a lot of moving parts to wear out, and in a moving vehicle this is going to be accelerated. And when failure does occur there are some rather scary failure modes (liquid air spill?)

What I'm not sure about is why you are exploring this idea? What problem does this solve for you that an off-the-shelf battery/inverter system does not?

That's probably right.

The overall goal this idea would accomplish is to provide heat, cold, electricity, and water, significantly extending the time the trailer could be boondocking/offgrid.

I plan to join the grey & black water tanks and use them only for grey water, and use a sawdust toilet.

The problems this idea would solve are:

dependence on plugging into the electrical & water grids

extra travel (once every 3 days with 2 adults & 3 kids) for getting water, dumping the tanks, getting propane; with this idea, regular shopping trips would be needed only for food & truck fuel (perhaps every 7 days when boondocking & not traveling much)

cooking with propane produces toxins in the living space

existing trailer heat, cold, electricity, and water systems typically do not capture & reuse waste heat & cold (from PV panels, inverter, battery, heater, air conditioner, refrigerator, freezer), but this system could do so

existing trailer heating, air conditioning, refrigerator & freezer systems may total more moving parts to break than would be involved in this idea's one unified heating & cooling system; this idea could be a gain in simplicity (but make all systems vulnerable to a single point of failure, unless the existing systems were retained and this system were added to them, which would be an incredible gain in redundancy)

older RV propane/ammonia absorption refrigerators are less efficient than newer compressor refrigerators, and in dual-mode refrigerators, involve converting PV electricity into storage in the battery into electricity (possibly inverted or buck-converted again) into cold

older RV air conditioners are less efficient than newer heat pumps/minisplits, and involve converting electricity into cold

LiFePO4 energy density is 110 Wh/kg; sand is 283 Wh/kg; liquid air is 121 Wh/kg

PV panels collect 10-30% of solar energy; thermal panels collect more than that and increase the efficiency of PV panels by cooling them; together they collect even more

PV panels' efficiency degrades slowly over time; thermal panels' efficiency may degrade at a slower rate

the next bullet points all center around this:  lithium is an expensive storage medium; sand & air are cheap; investing in the storage medium may be less economical than investing in the balance of system components

LiFePO4 batteries have a cycle life of around 5000 cycles then need to be replaced; sand in the sand batteries and air in the liquid air tank don't lose capacity over time; sand & liquid air batteries can be repaired with mostly inexpensive off-the-shelf components, except for the turbines/compressors/heat pumps

similarly, expanding the capacity of a LiFePO4 battery seems more expensive (buy cells) than expanding the capacity of a sand or liquid air battery (buy tanks)

LiFePO4 batteries are relatively heavy when 100% discharged, but liquid air batteries are relatively light weight when 100% discharged

lithium is more expensive, less renewable, more toxic, and more flammable than are sand and air