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Need to store surplus solar or microhydro energy, anyone?

 
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I'm currently working on a project which would allow storage of excess energy as heat in a thermal battery. Using a reversible chemical reaction of calcium oxide with water, the system will have a much higher energy density than water tanks. One possibility would be the storage of the the surplus photovoltaic energy in summer and use the energy for heating in winter.

Does anyone in this forum have a PV system that produces large surpluses in summer and a need for heating in winter? Could you give me an idea how much energy you have as a surplus?
Could you also think of installing additional PV modules to have free heating in winter? Please keep in mind that such a system can hardly compete with regular oil or gas heating, as it essentialy turns electricity (approx. 0.20 $ /kWh) in summer in heat  (approx. 0.08 $ /kWh using natural gas) in winter. Currently such a system is probable only viable in an off grid setting with surplus energy.

In a follow up post I will give you an overview over the system.

 
pollinator
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Location: Southern Arizona. Zone 8b
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fish bike bee solar woodworking greening the desert
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I'm not sure how practical that would be, especially for someone with significant heating requirements.  
My average winter-time heating requirements are pretty low, perhaps 50,000 BTU per day and perhaps 75 days a year when I need heat.  50,000 BTUs is roughly 14.65 kWh, per day, x 75 days.  Say 4 million BTUs or 1.2 MWh.  Someone further from the equator could easily require 5-10x as much stored energy.
We get a monsoon season during the summer so I actually produce more on an average day in February than I do during July.  
Spring and fall are when I produce the bulk of my surplus electricity, and many days I do produce 15-16 kWh surplus.  How much calcium oxide would be required for storing that much energy?  How much space would the stored chemicals require?

I come up with 110 tons of calcium chloride for my modest heating requirements, but perhaps I did the math wrong.

FWIW I'm not a good candidate for this type of system since I produce enough energy during the winter to run a heat-pump anyway.  But for someone that could use some type of long term heat storage, I'm guessing this would be prohibitively expensive.
 
gardener
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Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5
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Electricity is less efficient to store and get back out than if you store the heat itself. I've seen a lot of ideas over the years, for having a stone bank below the house for storing heat through the seasons.
 
Timo Bressmer
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Hmm, I get different results, somehow I end up with only approx. 2.5 tons of CaCl2. I found the following website quite useful: http://publications.tno.nl/publication/34607613/2MLhh3/trausel-2014-review.pdf (refer to table 1) .
It states an energy density of 1600KJ/kg for CaCl2, which means 1.6 GJ/ton. As 1 GJ is approx 1 MBTU only 2.5 tons are needed, with a price tag of 2.5* 116 € --> approx 356$.  (Beware, this was the second line in the table. There is also the option to use a different reaction based on CaCl2 with a lower energy density, which is the first line)

So probably the system for hydration/dehydration + storage is more expensive than the chemicals in this case.

Thanks for the mentioning of CaCl2, I wasn't aware of this. While it has a similar energy density + price tag than CaO per kg, it needs a much lower temperature for dehydration. But I guess I need to do some testing before deciding on the best chemical for this purpose.

Probably for you such a system really doesn't make sense if you can use a heatpump even during winter time. However, I still think the solution might be viable in colder climates.
Did you think of doing something else with your surplus energy? Or do you feed-in your surplus energy?
Ever thought of doing bitcoin mining (http://www.solidgreen.co.za/bitcoin-not-batteries-converting-excess-solar-power-money/)? (Actually I don't really support the idea of bitcoins. However converting surplus (green) energy to money seems like a reasonable idea).

 
Peter VanDerWal
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Location: Southern Arizona. Zone 8b
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Currently I'm grid tied and it doesn't cost me anything to 'store' my energy in the grid.  My power coop currently settles push vs pull up annually, and so far I've always pushed more than I pull.

I considered bit coin mining.  However, every time I run the calculations it would cost me more to buy the equipment than I could make using it for several years, even if the power was free.  Since my coop currently pays me 2.6 cents per kWh surplus I push, bit coin mining is a bust.
 
Timo Bressmer
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@Rebecca: Probably you are right that storing heat can be easily achieved using massive stone banks or big water tanks. However, this usually leads to low temperature heat that is emitted continuously. The use of a thermal battery makes high temperature heat (>100° C, depending on chemicals used) on-demand storage possible. So you could use this kind of heat for cooking, hot water generation etc. besides habitation heating.
 
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