Question: generating electricity from hot water...

Hi, so I'm working on a concept for a steam-driven electrical generator (steam generated by solar heat in a linear parabolic trough). That's fairly straightforward and has been done many times before, so completing the prototype for that will be "Stage 1" of my project. But I want to be able to generate plenty of electrical energy for storage or other uses so I want to generate steam even when the sun is not shining. So my concept is to heat water to near-boiling temperature at a faster rate than I can actually use it in a steam generator and then store this near-boiling water in insulated containers. This means I only have to raise the temperature a few degrees to turn that near-boiling water into steam.

So here's my question: If I use a propane burner (for example) to raise the temperature of the water to boiling and send that pressurized steam to the steam-driven generator, will this be a "net win" in terms of energy? I don't have to heat the water all the way from ambient, only just push it that last few degrees above boiling, so my thought is that I will be able to generate more total electricity than if I were to simply burn the propane in a regular, propane-fuel electrical generator. The reason I believe this is not due to some wishful-thinking violation of conservation of energy but, rather, due to the expansion of steam. Given that the water is already near boiling, the expansion effect should act like an "amplifier" on the propane burner, as though the burner is just unlocking the heat energy already stored in the water from being heated in the sunlight. Or perhaps I'm all out of whack. That's why i wanted to ask here in case anyone is aware of a system like this or knows where I can go to do more research...

To raise the temperature of one gram of water one Celsius degree takes 4.2 joules of energy. To convert that same amount of water to steam after it reaches 100 degrees requires an additional 2260 joules of energy. So getting the water to 100 degrees is a small part of boiling water.

The idea of relying on high water temps to produce steam reminds me of a new technology that captures energy from the Sun in the form of photo switches that change the nature of a molecule so it can store energy for long periods and release that energy in the form of heat when exposed to a catalyst. The reason your idea reminds me is that the ultimate temperature achieved is contingent on the temperature of the liquid the molecule is suspended in that is exposed to the sun and stored.

So for example: “If the liquid has a temperature of 20° Celsius (68° F) when it pumps through the filter, it comes out the other side at 83° Celsius (181.4° F)”. Water turns to steam around 100° C (212° F), so increasing the temperature of the liquid containing the photo switch molecule to achieve steam and then generate electricity seems possible. The energy from the Sun the process harvests is limited currently to the UV & blue spectrum, so improvement in extending further across the spectrum may improve the system where steam temperatures can be achieved without any additional input.

They are also working on solid materials that can absorb energy and release it as heat.

Fascinating.

I had a simpler observation, though: why not change the fluid medium to something with a higher heat capacity? My first thought was actually lead, to be honest, but there might be some homestead-scale molten salts that are more useful, or that neon propylene glycol stuff they put in the rink cooling systems in most skating/hockey and curling rinks.

I mean, you could superheat and pressurise water, in a series of check-valved, interconnected, insulated and buried pressure cookers, ready to be tapped at need. With a reserve such as that, it might be days before the sun needs to shine to provide power for you. This method is dangerous, but I think that's a given; done diligently, I don't think it's any more dangerous than any other plan.

But thinking about catastrophic failure conditions, I think I would prefer a molten lead or non-pressurised hot medium for the storing of heat.

Lead might be the easiest, actually. If you get a critical failure of a lead-based heat battery and steam generator, you might be looking at a lot of steam until the water is shut off. In a properly designed utility building, though, that's limited to the building, and as there's no pressurised vessel to rupture, the biggest mess would be had in reclaiming all the lead for reuse, and heating the exchange manifold to reclaim it all and assess the structural failure.

But you can do really warm water and have no pressure issues. Which I guess was the initial thought. No boom-squish, no molten metals or corrosive, explosive substances leaking.

Keep us apprised, and good luck.

-CK

Thanks for the replies! After posting (that's how my brain works... post first, think later), I realized that the "obvious" way to do what I want to do would be to use heat exchangers after dark to create a convection draft and pass this draft through fan blades to drive a generator. I'm guessing the efficiency would be pretty low, but better than nothing. In this way, a portion of the energy from the stored heat in the water can be recovered during the night hours or whenever the Sun isn't shining. That diagram almost perfectly depicts what I had in mind in the OP.

How about pico pumped hydro based on Earth's hydrological cycle? A passive solar heater could boil/evaporate water, pushing it upward to a condenser which feeds an elevated tank. That converts solar radiation to stored potential energy. Then, run it through a small water wheel connected to a DC motor/generator.

Douglas Alpenstock wrote:How about pico pumped hydro based on Earth's hydrological cycle? A passive solar heater could boil/evaporate water, pushing it upward to a condenser which feeds an elevated tank. That converts solar radiation to stored potential energy. Then, run it through a small water wheel connected to a DC motor/generator.

That's actually pretty good and just might work for the application I have in mind. I want to build one of these and use to capture large amounts of IR energy from the Sun. Allowing the steam to rise and then condensing it in an elevated tank would be a great way to store large amounts of potential energy for electrical generation when the Sun is not shining. And the whole system can operate at low pressure. I think I'm getting a concept now...