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Excess DHW heat to generate electricity
By DamonHD, Section Homebrewed Electricity Posted on Sat Mar 29th, 2008 at 09:47:25 AM MST
(Thinking how to avoid an 'oversized' solar thermal system boiling.)

Hi Y'all,
I had been thinking about oversizing a solar thermal system so as to be able to provide DHW all year round and possibly even some space heating. As pointed out any such system has the potential to boil like mad in summer in the absence of a decent heatsink such as a thermal store, especially when the household goes on holiday. And not many seem convinced about using a drainback system to avoid damaging boiling for various reasons.

Avoiding boiling: What if instead of a GSHP/thermal-store heatsink an air-source heat-pump was run in reverse to dissipate excess heat in the system. Or maybe something as simple as a solar-PV powered fan blowing air over external cooling coils. Could that avoid the boiling problem?
Could we do better than this and actually extract some of this waste energy from the ~60C temperature (DHW at 80C to air at 20C) gradient with (say) an Stirling Engine and a grid-tie inverter? The overall efficiency need not be large (and it won't be as a pure heat engine) and the grid is our infinite energy sink. My guess is the theoretical efficiency limit would be ~20% (80 / (273+80)) and maybe we might hope for half that, ie 10%. That would still be comparable to thin-film PV in terms of overall efficiency per square metre for example, which seems remarkable.
So (1) avoid boiling and (2) possibly recover some excess for the Grid.
What do you think?
Rgds
Damon

Excess DHW heat to generate electricity | 34 comments (34 topical, 0 editorial)

Re: Excess DHW heat to generate electricity (3.00 / 0) (#1)
by DamonHD (d@hd.org) on Sat Mar 29th, 2008 at 03:57:26 AM MST
(User Info) http://www.earth.org.uk/

Sorry, instant correction on the heat-engine arithmetic...

Hot end is top of DHW tank at 80C (ie hot enough to want to dump heat) and cold end is outside air at 20C in shade, so differential is 60C, so I assume heat engine efficiency is differential/hot-end, ie 60/353 = ~17% maximum.

Rgds

Damon

Re: Excess DHW heat to generate electricity (3.00 / 0) (#8)
by OzPete51 on Sat Mar 29th, 2008 at 05:23:51 PM MST
(User Info)

Hi Damon,
I am currently working on such a system, the object being to utilise VAWTS and Solar reflective heating panels to heat water and supply power to the grid.
Avoiding boiling is hopefully not going to be an issue, as my current idea is to use either antifreeze or high density salts to bring the boiling point of say 100-200 gallons of water to way over 100 deg c. This will then convert a small amount of a large cold water storage (up to 5000 gallons of water) to steam or near boiling water as required.
Using this concept - the antifreeze water is kept well below its boiling point by merely adjusting the flow of cold water that it is heating through coils. This should be simple to automate and should have minimal water loss through steam.

Your second point of using a stirling engine I believe is sound, but expensive. I believe steam turbines are much cheaper and can deliver higher payloads for the same dollars invested. You will probably find a 5HP stirling engine generator is much more expensive than a 5HP steam turbine generator.

The key measure of success for any such installation is pay-back. The more dollars spent - the longer to recoup and make money/free energy.

In short, as I have posted before, hot water has wonderful potential as the conversion medium for both VAWTs and Solar systems to link together and feed the Grid.

As always - Have fun in the sun - but don't get into too much hot water!

cheers
OzPete51

******* Original post
Hi Y'all,

I had been thinking about oversizing a solar thermal system so as to be able to provide DHW all year round and possibly even some space heating. As pointed out any such system has the potential to boil like mad in summer in the absence of a decent heatsink such as a thermal store, especially when the household goes on holiday. And not many seem convinced about using a drainback system to avoid damaging boiling for various reasons.

1. Avoiding boiling: What if instead of a GSHP/thermal-store heatsink an air-source heat-pump was run in reverse to dissipate excess heat in the system. Or maybe something as simple as a solar-PV powered fan blowing air over external cooling coils. Could that avoid the boiling problem?
2. Could we do better than this and actually extract some of this waste energy from the ~60C temperature (DHW at 80C to air at 20C) gradient with (say) an Stirling Engine and a grid-tie inverter? The overall efficiency need not be large (and it won't be as a pure heat engine) and the grid is our infinite energy sink. My guess is the theoretical efficiency limit would be ~20% (80 / (273+80)) and maybe we might hope for half that, ie 10%. That would still be comparable to thin-film PV in terms of overall efficiency per square metre for example, which seems remarkable.

So (1) avoid boiling and (2) possibly recover some excess for the Grid.

What do you think?

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#10)
by DamonHD (d@hd.org) on Sun Mar 30th, 2008 at 02:51:06 AM MST
(User Info) http://www.earth.org.uk/

Hi,

Thanks!

I've exchanged a few emails with a solar-thermal company here in the UK over the last 24 hours or so to get a rough idea of a system to cover all our DHW needs except in the depths of mid-winter. In this case a pressurised system with ~8msq of collector total on our east and west facing roof surfaces. Even then I might have to exchange my daily hot bath for a quick shower in December! This system is not looking cheap, but it is looking fairly routine in terms of well-tested technology.

That arrangement would have us rejecting 80% of the collected energy, some 40kWh/day solar energy wasted, in June.

Now, to cover the bulk of our over-night electricity consumption we'd really only need somewhere between 50W and at most 200W for lights and the TV, and my Internet-facing Web server. So bleeding out the energy slowly all night might be good and require smaller and quieter and cheaper equipment.

(I'm not attempting to handle the load spikes from the kettle, dishwasher, etc... We might as well take advantage of the Grid for that since it's there!)

It looks as if we could relatively easily accommodate a 500l DHW tank.

Supposing we stored excess energy by letting that tank reach 90C, and given that we only need DHW at ~50C (achieved with a thermostatic mixing valve for safety), the stored energy not needed for the DHW itself (and possibly having allowed a weekly excursion above 70C to kill Legionella, etc) is 40Cx500l = 40*500*4.2kJ = 84MJ = 23kWh which is about half of the excess energy that we'd otherwise reject in mid-summer, which means that we could just about discard that portion overnight and the remainder during the day.

We could discard that stored heat at about 2kW (ie 2kWh per hour) to keep up in summer, and at an efficiency of (say) 10% for a Stirling/Rankine/similar engine, that would be an electrical output of 200W, which matches the night-time loads described above. We might want to bleed off power at as constant a rate as possible overnight based on the length of night for the time of year, tank temperature, etc.

I'm wondering that if at 200W or less a solid-state (Seebeck) device might be possible. I'm only expecting/needing a conversion efficiency of 10% or so anyway. I still don't know about a grid-tie inverter, but maybe a Seebeck device might behave enough like solar PV (stable DC voltage) to use a small one of those.

In summer the otherwise-rejected 40kWh/day of energy might contribute as much 4kW/day of electricity to the Grid, a fair match to the 1.29kWp of solar PV of similar surface area already installed.

Someone tell me if I counted my fingers and toes wrongly again! B^>

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#2)
by PaulJ on Sat Mar 29th, 2008 at 04:46:56 AM MST
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1. Avoiding boiling is fairly important if you're talking about a pressurised hot water tank (I'm assuming DHW = domestic hot water?).

A steam explosion in a superheated hot water tank can be lethal, a system that provides adequate solar hot water in winter really needs redundant safety devices to operate safely in summer. At least a couple of PTR valves (in case one fails) as well as whatever you're using to dissipate excess heat - imagine a bird cr*ps on the PV powering the fan while you're away, you need something foolproof as a backup.

Here in Oz, latitude 37*, I have a solar HWS with a 330l tank and 2 panels each a bit over 1 sq metre - in summer I cover one of the panels, otherwise the PTR valve opens and wastes hundreds of litres of 80C+ water on a sunny day (serious drought here and we're on tank water).

2. I have a vague recollection of a TV show years (decades) ago where a cattle station in Australia was using 80C bore water to generate electricity, using the phase change in freon to run some sort of turbine, so you'd think you could do something useful with a 60C temperature difference.

Paul.

Re: Excess DHW heat to generate electricity (3.00 / 0) (#3)
by wdyasq on Sat Mar 29th, 2008 at 04:52:18 AM MST
(User Info)

Damon,
The second part is known as a 'Rankin Cycle' engine and the entire thing is waiting for someone to only figure out how to eliminate friction to make it practical. The 60C is a 'low delta'. The collection and dissipation devices will need to be quite large to overcome the inefficiencies. BUT-it has been demonstrated on a larger scale. AND - no, I'm not going to look up the examples.

Ron
Adventure is just bad planning." -- Roald Amundsen

Re: Excess DHW heat to generate electricity (3.00 / 0) (#4)
by DamonHD (d@hd.org) on Sat Mar 29th, 2008 at 06:27:41 AM MST
(User Info) http://www.earth.org.uk/

Thanks for the Rankin Cycle clue.

Googling already!

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#5)
by DamonHD (d@hd.org) on Sat Mar 29th, 2008 at 07:32:38 AM MST
(User Info) http://www.earth.org.uk/

A smaller version of the Rankine machine in yourownpower.com looks possible in principle, and I've seen a number of other suggestions that solar thermal plus heat engine of some sort should beat solar PV in terms of overall efficiency and cost.

So perhaps a very small geothermal power unit which is designed for these sorts of temperatures and low-delta-Ts might be the most practical to avoid much re-engineering.

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#6)
by bob golding (photoman290 at yahoo dot com) on Sat Mar 29th, 2008 at 08:28:27 AM MST
(User Info)

hi damon,
a much simpler and fail safe way to stop it boiling in the summer would be to have something like a ventian blind over the panels. you could operate it with one of those things used to open windows in greenhouses. simple, tested, and foolproof. might have some problems with wind but if you put a net over the whole thing,the sort used on scaffolding, that would catch all the bits.

cheers
bob golding

Re: Excess DHW heat to generate electricity (3.00 / 0) (#7)
by DamonHD (d@hd.org) on Sat Mar 29th, 2008 at 10:58:06 AM MST
(User Info) http://www.earth.org.uk/

It occurs to me that this scheme could also generate power at night from the stored heat in the tank as opposed to using batteries, which might make a oversized collector and tank more attractive.

Thanks to the positive noises and helpful clues so far, I'm pursuing this a little further and may do a full write-up. But I really have to try and find an (almost) off-the-shelf low-delta-T generator in the range approximately 50W to 1kW I think, including grid-tie.

I really like the idea of running CFL/LED lighting at night from a big hot-water tank! B^>

Rgds

Damon

Re: Excess DHW heat to generate electricity (3.00 / 0) (#13)
by DamonHD (d@hd.org) on Sun Mar 30th, 2008 at 08:52:58 AM MST
(User Info) http://www.earth.org.uk/

I'm starting a write-up on this idea (and some related work) here:

http://www.earth.org.uk/towards-a-LZC-business.html

since these might help me offset much of the CO2 from my day-to-day business activities which would be amazing if so.

I'm no longer trying to capture enough solar energy for space heating on my tiny property, but I'm attacking it from the other direction with more insulation, draft-proofing, etc.

Anyhoo, it's going to take me a while to finish this, but with luck it might be in a half-sensible state within 24 hours...

Comments welcome, as always!

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#14)
by spinningmagnets (velmis1450bc(at)aol(dot)com) on Sun Mar 30th, 2008 at 09:32:07 AM MST
(User Info)

I agree preventing losses is the best first effort, and best bang-per-buck (or Euro/Pound).

Once significant sealing of leaks is achieved, including additional insulation of windows, walls, ceiling, you may find the now accumulated breath of occupants will cause low oxygen, high CO2/humidity, and general staleness.

From reading the "German Passive House" standard, this was dealt with by:

Superinsulate and seal. (done)

Draw in fresh air through a buried earth tube to preheat air to ~50F. (not required at first, but obviously useful, can be added/expanded later)

Slowly pass a small volume per hour of incoming dry cool air through a heat exchanger, to absorb some of the heat of the outgoing moist stale air. (inexpensive and very effective)

Heat room air the remaining small temperature difference by any convenient method.

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#15)
by DamonHD (d@hd.org) on Sun Mar 30th, 2008 at 10:07:47 AM MST
(User Info) http://www.earth.org.uk/

Hi,

I hear all you say!

We have double-glazing everywhere, but have to fix bad drafts/leaks in the bathroom window (which has the mitigating virtue of reducing condensation at the moment).

In general we're a still a long way from sealing everything tight, but I am definitely thinking of some heat-exchanged fresh air if we get close...

I am aiming for body heat and solar gain to cover most space heating needs eventually.

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#33)
by scottsAI (user name at eml dot cc) on Thu Apr 10th, 2008 at 06:19:03 PM MST
(User Info)

DamonHD,

Ventilation.

Wind induced ventilation, no motors, in / out vents form a heat exchanger. Humidity is not preserved.
To preserve humidity is called entropy ventilation. More costly.

Have fun,
Scott.

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#9)
by scottsAI (user name at eml dot cc) on Sat Mar 29th, 2008 at 09:33:23 PM MST
(User Info)

Hello DamonHD,

I started down this road couple years ago. Lets do some math:

100watt load to run for the night = 12 hr.

Assumption: you can superheat the water 10 deg.
Low temperature heat engines may achieve 10% efficiency.

3.4BTU/watt-hr * 12 * 100w = 4,080BTU * 10 / 10 = 4,080BTU needed.
Water has 1 BTU per deg per lbm or 4,080 lbm / 8.35lb/gal = 488.6 gallons

Two golf cart batteries supply more power discharging to 50%.

You can build a larger cheap tank, the other equipment cost are higher than the equivalent solar electric would be? I decided to cool the solar panels to improve their efficiency for the best bang for the buck.

Use the waste heat to melt the snow on my drive way and side walk. Assuming the homes heating needs are met.

Have fun,
Scott.

Re: Excess DHW heat to generate electricity (3.00 / 0) (#11)
by DamonHD (d@hd.org) on Sun Mar 30th, 2008 at 04:14:39 AM MST
(User Info) http://www.earth.org.uk/

Yes, in this case I'm simply attempting to (a) first cover DHW requirements as much as possible and then (b) using the 80%+ waste/excess heat in summer to do something useful rather than boil the system and make me grit my teeth.

100W of Seebeck devices at ~5% efficiency didn't look like being horribly expensive from the first Chinese site I ran across...

So I guess the nearest obvious apples-to-crabapples comparison is whether the extra water tank capacity that I might put in to support slow energy release overnight is more expensive than the batteries that I might use instead, but the value of that excess capacity in winter is storage of heat for dull days and the generation side-benefit is a lucky bonus.

(Oh and heat leakage from the tank in winter might help take the edge off space-heating requirements.)

So really the point is if the Seebeck devices and supporting nonsense cost more than batteries. Which is I think the analysis that I shall attempt to apply.

Thanks for your pointer!

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#12)
by scottsAI (user name at eml dot cc) on Sun Mar 30th, 2008 at 08:51:42 AM MST
(User Info)

Take a look at papercrete for the tanks insulation, not its structure.
Found a liner that can take 300 deg F, can make 1000 gal tank and bigger for about $1 per sqft of liner. I would consider putting the tank in the ground, cant flood you house that way.

Have you looked at PEX for the tubing?

I have designed a zero energy home with the solar features built into the structure, cost adder is 10% above standard construction. No energy bills. Looking for someone with a workshop to test it out. don't want to build a complete home without testing the ideas.

Have fun,
Scott.

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#24)
by DamonHD (d@hd.org) on Tue Apr 8th, 2008 at 02:10:19 PM MST
(User Info) http://www.earth.org.uk/

Hi,

I only just saw this post: sorry to miss it.

I can't seem to get any consistency in U-value for papercrete. Do you think it would be better or worse than foam?

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#28)
by scottsAI (user name at eml dot cc) on Tue Apr 8th, 2008 at 05:42:05 PM MST
(User Info)

DamonHD,

Check out http://www.livinginpaper.com/index.htm

Papercrete R2 to R3 per inch, depends on the mix. (how much sand, concrete etc)
Foam is R5 to R7 per inch, with a cost of $4 to $6 per cubic foot.
I can get 5 tons of paper for free, the Portland cement will cost (?), I saw a 80lb bag for $6 last year.

The papercrete is so much cheaper, use twice the thickness to get the same value or more.
A local recycle center will not take foams without a recycle label on it. You could request some and grind it up adding it to the mix for greater R value if important to keep walls below a certain thickness.
Many things you can do, let us know what you have in mind. Glad to help.

Have fun,
Scott.

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#29)
by DamonHD (d@hd.org) on Wed Apr 9th, 2008 at 01:32:48 AM MST
(User Info) http://www.earth.org.uk/

Hi,

Thanks for the link! Will read shortly.

Space is a huge constraint for me, but I know that I'm going to need a mix of materials in any case if I do it.

The thing is, by my estimate, I need amazing insulation since I would need to try to keep losses from the tank at or below ~10W (yes, ten Watts, not W/K or kW or 'what'ever) even with an outside temperature of ~10C to retain most of the heat between seasons.

Digging a cellar under our porch at the front of the house is looking like one solution providing that the local maze of drains/sewers doesn't pass under it.

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#32)
by scottsAI (user name at eml dot cc) on Thu Apr 10th, 2008 at 06:14:30 PM MST
(User Info)

DamonHD,

To hit 10w for any tank of size is impractical.
Vacuum insulating panes might do it. Very costly.

Retaining heat between seasons look into ground source storage.
Dry well, store the heat in the ground, large area can store huge amounts of energy.
This is my plan for driveway heating, store the heat below. Bring heat up when it snows.

In another area store the cold from winter for use in summer cooling.
Or do like the pioneers did, ICE house, even thin insulation in ground a large block if ICE can run your fridge and AC all summer long.

Have fun,
Scott.

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#34)
by DamonHD (d@hd.org) on Fri Apr 11th, 2008 at 01:36:19 AM MST
(User Info) http://www.earth.org.uk/

Yes, 10W is impractical.

However, I had a slight brainwave. Instead of a 5t tank losing 10W (10% loss of stored heat over 6 months), why not use a 10t tank losing ~100W (~50% loss of stored heat over 6 months)? The extra energy to put in the tank is readily available in the summer, and I need only ~10% of the insulation, ie something like 50cm instead of 5m+.

The downside is that we'd definitely need to dig down to accommodate a large enough tank as we just don't have lots of ground space. But we might not have to dig much.

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#16)
by clarsondd on Sun Mar 30th, 2008 at 06:12:20 PM MST
(User Info)

here is a thought,

using parabolic trough collectors. i'll just pick a number such as 4.

set up the plumbing such that in the winter the working fluid passes through the one time, much the same as in a flat plate collector.

then during the summer, probably using some values, set up the plumbing such that the fluid passes through all the collectors, i.e. output of panel one is feed to the input of panel 2, output of panel 2 is feed to the input or panel 3, etc.

set up this will the panel will produce low grade heat to be used for space heating.
Then, in the summer the panels will produce steam. Use the steam to run a steam generator. use the condenser heat to provide DHW and feed the electricity to the grid. Using steam will double, maybe even triple your efficiency.

something ala:
http://engineersformyanmar.blogspot.com/2007/08/how-to-build-solar-generator.html
http://www.popularmechanics.com/science/earth/4232571.html

i saw alot more info on the web before, maybe you can find more with some searching.
here is one, but the links don't seem to work: http://web.mit.edu/amym/www/pages/lesotho_pics.html

Re: Excess DHW heat to generate electricity (3.00 / 0) (#17)
by DamonHD (d@hd.org) on Mon Mar 31st, 2008 at 02:55:04 AM MST
(User Info) http://www.earth.org.uk/

Hi,

Thanks for that thought.

Now you have to understand that (a) in winter my entire roof area does not receive enough sun for space heating and DHW and (b) I don't think I'd get planning permission for parabolic troughs on the roof especially given that we're under the Heathrow flight path and someone would assume that I was building a DeathRay(TM)!

But I'll have a think.

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#18)
by The Crazy Noob on Tue Apr 8th, 2008 at 09:39:33 AM MST
(User Info) http://users.telenet.be/cool_things_with_fresnel_lenses

Some more things to google for:

The seebeck device you mention, have you tried googling for "peltier"? It might give you more hits and it is the same thing if you run it in "reverse" (I think).

For going grid-tied with a type of heat engine: you don't need an alternator/inverter setup; you can directly link the shaft of the engine to a plain induction motor. If the heat engine turns slightly faster that the induction motor normally would run (~10% faster, called the "slip"), it will automatically back-feed to the grid. Keywords: "induction generator".

---
"If the honey bee goes extinct, man would have no more than four years to live." -- Not A. Einstein, nevertheless something to think about!

Re: Excess DHW heat to generate electricity (3.00 / 0) (#19)
by DamonHD (d@hd.org) on Tue Apr 8th, 2008 at 10:31:35 AM MST
(User Info) http://www.earth.org.uk/

I have searched for the reverse device as you suggest, but the most promising was not speced for generation. But I'll no doubt have another look... B^>

As to the induction generator, that's well beyond my skills and will not be legal for me to connect! And actually it's getting anything to turn (eg the Rankine bit) that I don't have a clue on. As it happens I have struck up a conversation with someone who may be able to help...

But I'd like to keep the thing as cheap, legal, and off-the shelf as possible!

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#20)
by ghurd on Tue Apr 8th, 2008 at 10:56:50 AM MST
(User Info)

Maybe something a bit simpler?
You have the server or router set up with a grid fed UPS, and decent sized batteries?
Maybe dump into those batteries, reducing the grid power used to charge them. It must depend on how the UPS works.

I have one that is a small battery charger, to a small battery, to a small inverter, and that's about it. Hardly state of the art. Any power solar put in the battery would be that much the gid didn't have to supply. Inside-out grid un-tie?

BruceS is doing something about like that, with my kit and a diode.
G-

Re: Excess DHW heat to generate electricity (3.00 / 0) (#21)
by DamonHD (d@hd.org) on Tue Apr 8th, 2008 at 01:00:03 PM MST
(User Info) http://www.earth.org.uk/

Hi,

The only UPS I have for my server is the internal battery, but yes, I am thinking along those lines too.

I think that I'm about to order up one of the Seebeck generators to test out at about 2% efficiency for my 60K--70K delta-T.

I hope to be able to generate enough power to run the forced-air heatsink fan and have some left over nominaly to charge a battery for this experiment. The device itself is rated for a maximum of about 13W out, though not under my conditions I suspect.

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#22)
by ghurd on Tue Apr 8th, 2008 at 01:19:44 PM MST
(User Info)

Someone said, IIRC, they only last 6 weeks under 24/7 use?
They considered it a consumable worth the money, like water used in a battery, IIRC.
Scott? Roger?

It was a major burst to my tiny bubble at the time. That I recall very clearly.
G-

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#23)
by DamonHD (d@hd.org) on Tue Apr 8th, 2008 at 01:57:42 PM MST
(User Info) http://www.earth.org.uk/

Hi,

The Seebeck generators? Well, I hope not: I shall be putting significant $$$ down, though only prototyping at this stage, so I guess it could be considered disposable.

This is one of the suppliers I'm looking at:

http://hi-z.com/

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#25)
by ghurd on Tue Apr 8th, 2008 at 03:32:08 PM MST
(User Info)

'Propane generator' and 'Alaska' come to mind, as terms to narrow the search.
Something like that was tested as very good.

There is/was a California company selling items made in India. Kero lanterns that powered a radio.
They bundled the lantern, Peltier device, and radio as a single unit. Earthquake kit nonsense.
They had an add-on generator for 'white gas' or Coleman lanterns.
They could have gone under, and I believe they did.

I'm lost on the 2% efficiency and 60K Delta-T. Typo?
Sounds like a worn out steam engine.

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#26)
by DamonHD (d@hd.org) on Tue Apr 8th, 2008 at 03:43:09 PM MST
(User Info) http://www.earth.org.uk/

Those applications you suggest have a much bigger temperature drop (delta-T) from the heat source to the heat sink.

I'm sure you know this, but the maximum efficiency of extraction of such a system is very roughly the delta-T divided by the higher temperature in Kelvin. (It's a heat engine.)

So my 60C--70C delta-T system is doomed never to be more than about 10% efficient by my untrustworthy mental arithmetic. If I actually achieved anything like that 10% then I could generate electricity from solar DHW about as efficiently per square metre of collector as putting your hated thin-film PV in place of the solar thermal collectors, but all this while still leaving the hot-water system running as before, just boiling a little less or not at all. Not bad for a 'free' byproduct.

So, I'm prepared to waste a little time and money seeing how close I can get. If it looks plausible then it'll encourage me to get a solar thermal system in sooner rather than later.

Rgds

Damon

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#27)
by ghurd on Tue Apr 8th, 2008 at 03:55:13 PM MST
(User Info)

My bad. I crossed one post with the other.

Still. Look at/for the Alaska / propane thing. Good heat to watt Peltier info.
The unintentional change to "Vista" means I have no clue where it is.

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#30)
by DamonHD (d@hd.org) on Wed Apr 9th, 2008 at 01:33:53 AM MST
(User Info) http://www.earth.org.uk/

Roger Wilco!

[ Parent ]

Re: Excess DHW heat to generate electricity (3.00 / 0) (#31)
by DamonHD (d@hd.org) on Thu Apr 10th, 2008 at 01:36:39 PM MST
(User Info) http://www.earth.org.uk/

OK, I've ordered some HI-Z Seebeck generators to experiment with.

I was told that given the low temperature that I am going to operate them at they should last 'several years'.

Rgds

Damon

[ Parent ]

Excess DHW heat to generate electricity | 34 comments (34 topical, 0 editorial)

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