solar - storing superheated water 1

[bootup]

My brother and I are looking at ways of storing solar energy in high temperature/pressure water. He's currently using solar PV arrays, but the energy densities of water in the 600F range look very attractive compared to batteries. What I am not able to decipher from the online steam tables (my background is mostly software) is the effect of the container size and water volume on pressure. For example, if we put 10 gallons of water in an 11 gallon container and heat it to 600F, what is the resulting pressure and how would it change if we used a 12 or 13 gallon enclosure instead. I'm guessing more room for water to flash to steam would mitigate pressures. And would the larger container adversely effect our ability to extract work from the water?

Thanks for any pointers

 

[IRstuff]

As general rule, you want to use a storage medium that provides the highest "quality" energy. Batteries do that.

A thermal storage is limited in the amount of extractable horsepower. Additionally, how are you going to keep that super-heated state over, say, one month?

TTFN

 

[bootup]

Aerogel blankets are becoming affordable. We don't need to store the water (without reheating) for more than 48 hours. In this case, the sun is plentiful.

 

[davefitz]

Depending on what temperature you ultimately need the heat at , you might want to investigate the use of eutectic salts at a lower temperature , in lieu of hot water.

The pressure vessel will need a relief valve, set at the design pressure of the pressure vessel. If you fill the vessel with water and heat it, the water will "swell" and the presure will rapidly increase to the design pressure, and begin to weep thru the relief valve. As the temperaure rises to the saturation temperature, then steamwill begin to weep thru the relief valve.

One advataage to a hot water system is taht it can circulate naturally, if the system componenst are sized adn arrangeed properly. But if you only need heat at about 212 F or lower, I qould use eutectic salts.

 

[btrueblood]

bootup,

Yes, you need ullage space in a tank to avoid the thermal expansion of the water causing the tank to burst, as davefitz pointed out. To find out how much ullage (also sometimes called "headspace") you need, look at the ratio of density of the cold water to the density of saturated liquid water at the 600F temperature. This ratio is how much the liquid will expand; multiply the ratio by the volume of cold water to get the tank volume required to "just barely" contain the hot water. Add a lot of safety margin. Have a qualified PE do the final design your tank for you, steam pressure vessels at 600F are not something you want to "let go" anywhere near you.

 

[JStephen]

Water at room temperature is about 62.4 lb/cuft. Saturated Water at 600 degrees F is 42.3 lb/cuft. In order to avoid very high pressures, you need to have enough empty space for that degree of expansion. (IE, 14.75 gallon tank with 10 gallons of water, with air evacuated).

If you have a slightly bigger tank, say 16 gallons, you'll have a mixture of saturated vapor and liquid at 1543.2 PSI. Increasing the tank size won't affect the pressure until you get up to a tank 166.9 gallons capacity. At that point, you'll begin to wind up with a tank of vapor only, with the pressure dropping off with increasing tank size. In between 14.75 and 166.9 gallons, changing the tank size will give you more vapor and less liquid, all at the same pressure of 1543.2 PSI.

Building a tank and piping to hold water at 1500 PSI is not going to be cheap, if that helps any.

You might also check on energy conversion efficiencies. If you need the energy back out in either mechanical or electrical form (and not heat), then the efficiency of that conversion is going to be around 30% or less. This and the cost of storing high-pressure water may make the battery alternative look a whole lot more promising.

 

[bootup]

Thanks for the information and explanation.

I understand Carnot cycle efficiencies are limited to about 30%. However, that only takes into account the temperature differential. We also have a large pressure differential. I'm not sure how to calculate the work energy that will impart if we end up driving a steam engine.

 

[JStephen]

If you're just using the pressurized water to store heat, then the pressure won't do you much good- siphoning off vapor to run a steam engine will drop the temperature and pressure remaining in the tank fairly quickly.

 

[EdStainless]

You should look at using eutectic salts for storing energy. I know that Solar 1 used this method and some geothermal facilities do also. These systems are very good for energy smoothing. Most of the info is available from DOE because so much of the work was gov funded.

If you want to talk about materials of construction I would be glad to.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[bootup]

Well, the primary goal is to produce electricity from the stored energy. Are eutectic salts were good in that area? I was looking at this more from the point of densities in a fixed area and weight. I did some calculations that told me water at 600F-650F held 10 to 20 times as much energy per pound as lead acid batteries. Even if I can only reclaim 30-40% of that during regeneration, I'm way ahead of the batteries for storage in terms of size and weight. Also, driving inverters from the batteries to produce AC is only about 90% efficient. The other factor I wanted to consider is that batteries wear out. It's a costly component with toxic elements that require recycling.

 

[IRstuff]

You should be looking at energy per VOLUME, not mass.

Additionally, any benefit there might have been is further reduced by the mechanical complexity of the pressure vessel, insulation, plumbing, etc.

TTFN

 

[btrueblood]

Not to mention the complexity of steam turbine, alternator, control valves, etc.

 

[bootup]

I'll have to look back at my calculations, but on a per-volume basis I think it's still 10 times as dense.

 

[JStephen]

Keep in mind that the pressure vessels and equipment will probably weigh more than the water that is in them.

 

[bootup]

Yes. Possibly up to 2-3 times the weight.

 

[zdas04]

To get electrictiy from water you have to (1) heat it to boiling; (2) dry the steam; (3) use the steam to drive an engine (turbine has the best energy conversion) connected to some sort of alternator; (4) condense the steam; and (5) pump it back to your steam generator. Call it $3 million for code equipment and installtaion. You can do it in a pretty small space, call it 3,000 sq ft with 30 ft ceilings. Don't forget to budget a licensed stationary engineer to run it ('round the clock, 'round the caledar) and lawyers to get the zoning variances.

My experience with turbo-alternators has been that even very small ones outrun the energy collection capacity of solar panels very quickly. It takes a lot of panels to make a small amount of steam, and what is your energy source at night?

If you must do this, then you could use a gas-fired boiler and go to the panels for superheat (steam will only become superheated if a coherent gas-water interface is absent).

I'm really confused about why you are looking at "energy per pound" and then comparing the infrastructure of a lead acid battery to the working fluid of water. When you add the infrastructure of vessels, piping, generators, valves, and insulation, the battery is probably at least 10-20 times the energy density of a steam plant. If you look just at working fluids of the two, you get even more impressive results. The "working fluid" of the battery with PV panels is electrons and not enough of them to weigh very much - I would expect energy per pound of electrons to be pretty high.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[EdStainless]

I see that the biggest factor is one of scale. For small and medium instalations there is no doubt that if you want electricity you are better off with PV in the first place.
For large instalations, where space and capital costs are big to start with, then turbogenerators have the edge.
In some geothermal systems they use steam, but many use ammonia or butane to run the turbines. These lower boiling point fluids can be superheated with lower grade energy sources. You get system efficency without needing high temperatures. But, they are a pain to work with.
The old Solar 1 project stoured energy in molten salt and then produced steam and power around the clock.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[JStephen]

Whilst browsing around here, I actually ran across an "Energy Storage Engineering" forum, which might get you another worthwhile answer or two. It doesn't seem real active, but worth a try.

 

[GenB]

I red topics on hot water turbines, interesting subject,
I never knew if it worked or not.
It looks to me that hot water at 400psi and 350-500 degF can run the turbines.
You can raise the temp of the water "hi-temp water boilers"
If of interest, follow the link at

http://www.steamequip.com

and send an email.
If you can raise the temp that high using the sun-power, I am interested and I can make the Coded Vessel for you.
As per the power turbine, you will have to research the patent which probably already exist.
GB