Advice request: Combined Heat and Power Solution for fully off-grid

Adam_L · January 2018

Hi All, I’m new to these forums and would very much appreciate your help. I'm part of a team participating in the National Science Foundation Innovation Corps – a program helping scientists take their innovations out of the lab and into the market. We think the technology our team is developing may offer a lot of value to folks living completely off-grid and we’re trying to better understand the pressing needs and interests of this community.

Would any members of this community be available for a 15 minute phone call to help us understand the challenges of making your own power and whether what we are working on might make life easier.

Please comment below if you’d be willing to help and I’ll contact you privately to schedule a call.

Many thanks,

Adam

BB. · January 2018

Adam, what may be helpful to you--Give us a design you would like to implement--And we can go through the thought and design process (paper design) pretty quickly so you can see how (at least some of us) get a system designed and running.

More or less--We try to focus on loads/energy needs first. In the US, nominally, solar power usually ends up costing ~$1 to $2+ per kWH for self installed systems. And to be honest, two big issues with solar power system costs.. Batteries (last 3-8 years typical) and electronics (last ~10+ years before replacement). And, the "chain of losses" (81% panel eff from marketing numbers, 95% efficient charge controllers, 80% efficient flooded cell lead acid batteries, 85% efficient inverters ~52% overall panel to AC load efficiency).

And many areas that have as low as 1-2 hours of sun per day in winter (much of northern Europe--need genset to make power in winter)... 3-4 hours of sun makes solar system much more efficient.

And many people try to use wind turbines--This is one place were DIY (do it yourself) Wind Turbines tend to be better than off the shelf systems. Any wind turbine system is dangerous and needs to be away from occupied buildings/grounds (tossing blades, ice, nacelles falling from towers, tower collapses).

And the random fires (electrical, battery, genset, fuel, etc.).

-Bill

mike95490 · January 2018

i'm open to a phone call, but would like to see the ideas and results shared by all, not buried in a research project that requires a $4k subscription to view.

As an engineer that planned for several years before buying a single thing, I'm interested to learn what new stuff came along in the last 6 years, that I've not heard of. (dont you dare mention solar roof tiles - until you have solved the lifetime, wiring and heat issues)

Estragon · January 2018

I also would like more info posted here about the idea. The OP may be well meaning, but it comes across to me a bit as trolling for sales leads.

Adam_L · January 2018

Hi Folks, thanks for the responses. I completely understand your caution, I hate spamers and telemarketers too, I'll try my best to convince you that's not what we're doing.
We're not selling anything right now, but we do plan to make a commercial product in the mid-term. We're a group of researchers with a lab-scale proof of concept of an alternative way of making electric power from a liquid fuel. We have a theory that it might be useful for folks living off the grid, but we want to go out and talk to the people know actually know what they need before building the technology into a product.
Does that address concerns? I'll PM those kind enough to respond already

BB. · January 2018

In general, off grid solar folks use gensets for "dark" weather. On a very stormy/dark winter day, the solar panels can generate as little as 1%-10% of rated energy vs a sunny winter day (virtually nothing).

When sizing the system, for flooded lead acid batteries, we generally size them for 2 days of "no sun" and 50% maximum discharge. 1-3 days of storage is the maximum "range" that is typically useful.

50% maximum discharge, longer cycle life, lessens the chances of going to zero volts/reverse charging of weak cell(s).

The 2 days of storage--Not only lessens the use of a genset for "short storms", it gives a "balanced" battery bank for the loads. Smaller systems generally have smaller peak loads vs larger systems. As an example, a good rule of thumb for maximum discharge and charging current (flooded cell lead acid batteries) is 1,000 Watts per 100 AH of 48 volt battery bank capacity (or 250 Watts per 100 AH of 12 volt capacity).

The liquid fuel idea... If is something that has to be "shipped in", then besides figuring out how to use the fuel (internal combustion engine to generate electricity, open burner for stove/hot water/heating), there is storage. Gasoline, stored ~1 year max with fuel stabilizer in sealed containers (need to use/recycle after that time, MTBE, Alchol, etc. additives are an issue). Diesel can keep longer with stabilizer--problems with water and algae growth. Propane, long term storage is great.

If there was a solar electric to liquid fuel reactor--That would be interesting. A "balanced" solar system design that gets you through the sunny days of winter will generate 2x as much power as needed in summer. Batteries are only "good" for short term energy storage (a few days). Hydrogen is a tough fuel to store (high pressure, hydrogen embrittlement of metals, lots of storage for little BTU, etc.).

The ability to create on site a stable/usable liquid fuel (at lower pressures, if needed) with excess solar energy would be very interesting.

In general, off grid solar electric energy is expensive ($1-$2 or so per kWH), and other fuel sources (liquid, propane, wood, even solar thermal) is a better use of $$$. The use of heat pump mini-split systems has made solar electric use for HVAC economically doable. As well as heat pump water heaters (~2-3x more efficient for heat pump vs resistive electric water heating).

-Bill

Adam_L · January 2018

Thanks for all that information, Bill. What problems do people have with their generators? And has anyone looked into doing combined heat and power or cogeneration - where you use left over heat from your generator for water or space heating?

Estragon · January 2018

If it could be done economically on a smallish (eg ~10kw power, 20kw heat) scale, combined heat and power would certainly be of interest. Fall, winter, and early spring are difficult for solar, and also require heat. The genny might be run 3-4 times a week in these months, and maybe not at all in spring and summer.

I currently do a very crude version of CHP by routing the exhaust of my diesel though buried recovery piping under the genshed. All it does is warm the ground to make starting the generator easier. I have also considered plumbing a heating coil into the cooling/radiator controlled by an air temp sensor. At 4kw power output, it might get me 2-3kw heat.

As Bill noted, the main problem with generators is transporting and storing fuel.

BB. · January 2018

Adam,

10 years ago, Honda had a co-generation system:

http://forum.solar-electric.com/discussion/5427/heat-harvester-freewatt-cogeneration-system
http://news.honda.com/newsandviews/article.aspx?id=4880-en

You can read through the old thread... Basically 1.2kWatts of "waste" elecricity (to grid-tied utility panel) and 11 kWatts of heat (3,224 Watts of electrical equivalent).

The standard system (as of that time) was setup to cycle the natural gas powered generator for the heat, and run the "waste" electricity to the grid.

This Honda blurb seems to indicate that they are still developing (and selling?) a 4th generation unit these days:

http://world.honda.com/powerproducts-technology/cogeneration/

Anyway--There are always issues with co-generation... Especially off grid Co-Gen.

On grid, you can run the engine when you need heat, and dump the "excess energy" to the grid (and if you have some form of net metering, you can be paid ~$0.05 to $0.30 per kWH (depending on state/utility/rate plan).

With off grid systems, you generally run the generator when you need to recharge the battery bank and/or when you need to run large loads (shop, tools, compressor, recharge battery bank during poor sun, etc.).

Storing of electricity into a battery bank is generally something like a 3-8 hour run-time every few days (mostly in the morning is recommended). Not when you need heat. And in sunny weather (something like 9+ months of the year), the genset is not even used.

Also, a genset is usually pretty lightly loaded (10-25%?) for typical loads... They are only "heavily loaded" when running battery charging (~60-80% loading is optimal design).

And lightly loaded gensets (internal combustion engines) through off significantly less waste heat. The type of engine also matters--A gasoline engine generally runs (very roughly) 50% fuel flow rate at 50% or less of electrical load (more waste heat). A diesel (Otto Cycle) engine runs much less fuel flow at lower output levels (25% or less--just a guess). So less waste heat for diesel (and why people buy diesels is for their efficiency).

In some (especially older) diesel installations, they would put load banks on diesels to ensure that they operated at 60%+ electrical loading to keep the diesel "happy" (higher pressure reduces cylinder glazing, reduce carbon buildup, prevent "wet stacking"). Some folks here say that modern diesels have less/no issues like that any more--But I would still suggest 40-60% minimum loading in designing a system (I am not a diesel mechanic or engineer).

Running a gasoline or diesel genset without pulling much electricity will not generate much waste heat (you try to pull co-gen heat, and the motor will run too cold--Diesel cars are known for this problem, and so are Prius cars too--Little "waste heat" for cabin heat in cold climates).

And there are always the "gotcha" problems. Over cooling the motor. Over cooling the exhaust (water/corrosion, condensing carbon). Exhaust leaks into building, air cooled engines with gasket/exhaust leaks). Where to "store" heat for evening/nighttime use...

Does not mean that people do not try (I believe that is a carbon monoxide detector on the floor next to the genset--although, that could be a fire alarm/smoke detector instead--not a great safety solution).

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If you try to get a lot of heat at high temperatures (i.e., wood stoves, boilers, etc.)--That is a huge set of (explosive) dangers in its own right (water/steam boiler explosions, improperly designed/installed wood stove heat recovery systems, etc.).

-Bill

Dave Angelini · January 2018

There are also quite a number of my clients in the southwest US that live quite easily without generators. They have systems large enough, loads that are discretionary, and the location to support offgrid in winter. I once owned a generator

mike95490 · January 2018

I played guinnie pig, and had a nice long chat with them, some very interesting ideas.

Adam_L · January 2018

Many thanks to mike95490 for the call and to BB for, wow, a textbook on off grid chp. We're working on a propane-fired, solid-state generator (think solar panel, but with heat as the input rather than light). It would need to be cooled and the cooling water could be used for heating requirements. It would be small wattage relative to a diesel but designed to run for longer periods. The problem of what to do with the heat when you want power but not heat is real, but we're thinking of some control logic and a small outdoor radiator to reject excess heat. Please rip the idea apart, what issues are we not considering, why won't this be useful to anyone?

BB. · January 2018

Adam, essentially a propane "fuel cell"? As I understand (certainly no expert), generally fuel cells are (more or less) constant power devices and don't really have surge capabilities. For typical "random loads" (vehicles, buildings, pumps, etc.) they need an energy storage device (aka battery) or other energy source (genset) to maintain arbitrary loads (or a much larger/more expensive fuel cell?)...

Propane Fuel cell from 2006 article:

https://www.sciencedaily.com/releases/2006/11/061113170844.htm

The "surge" and arbitrary loads are also an issue for off grid power systems... Smaller battery banks (and smaller/fuel efficient gensets) do not have a huge amount of surge capability either. To get around wanting smaller (less costly) power systems for off grid use, I/we suggest to look at loads. Typically these are smaller pumps (aka "slow pumps") and VFD (variable frequency drive) technology electric motors that allow soft start and to run at less than full speed--When lower power levels are "OK" (water pumping in less than full sun, running an "inverter" based mini-split AC/Heat pump at low power most of the day to keep home cool/pre-cooled vs running at high speed in the evening when everyone gets home). Of course, the efficient and smaller VFD pumps (with Permanent Magnet Motors) are more expensive (where to put money--In conservation or in power generation--Generally, I suggest conservation is money better spent than in larger generation and storage).

Is your propane "fuel cell" something like this (high operating temperatures to avoid expensive noble metals for electrodes?):

https://en.wikipedia.org/wiki/Solid_oxide_fuel_cell

The old question of how to manage thermal energy (dump it as waste, or use it to heat an insulated tank of water--Or even use heat pumps to freeze water during the day (or over night for cheap utility power) and extract heat for cooling when needed). Or just dump waste heat to the outside with a radiator--when not needed elsewhere. Certainly works--Just cuts down on the overall "fuel efficiency" of the system.

Another concern... In off grid homes and cabins, there are significant theft/damage issues (nobody around to report prowlers, pot shots, etc.)... Keeping installation costs low/reasonable (with 10 year component life) vs an expensive 40 year life system (vulnerable to vandalism, expensive materials like copper, or even steal the unit and move elsewhere) could be an issue too.

-Bill

mcgivor · January 2018

This must be a thermoelectric generator (TEG) or Seebeck generator of some design, the advantages being quiet, as opposed to internal combustion, along with no moving parts, unless active cooling is required, makes it an interesting idea, especially for overnight use, however, correct me if I'm wrong, but aren't such generators very inefficient, typically 5-8%, excluding any heat recovery?

BB. · January 2018

If it was a TEG, yes very inefficient (typical used for low power systems on mountain tops, remote areas, etc. or stuff it with plutonium and launch it in a satellite or install in remote regions and abandon like the USSR did decades ago to a few places using RTGs).

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

I don't think Adam is talking about a TEG (something like 50 watts is typical). Some sort of fuel cell and higher wattage and efficiencies?

-Bill

mcgivor · January 2018

You're probably right about it being a SOFC rather than TEG, but TEG is used in applications up to 5kW, have seen them in North West Territories Canada, not the radioisotope versions, but propane, how, if so inefficient, do manufacturers stay in business, unless they've improved efficiency, these were the type, http://www.genthermglobalpower.com/products/thermoelectric-generators-tegs I'm no expert just something I've seen.

Mangas · January 2018

Generator here is standby.

Off grid photovoltaic system is large enough to deliver all power requirements 24/7 with significant storage capacity headroom.

Dave Angelini · January 2018

A good project to use up grant money. Pretty much yawning on this when there is so much to do to protect the 3 big grids in the US.

Good luck!

Someone wrote me about why I said the southwest was easy to live offgrid. I meant to add "and similar places" in the world that are not subject to coastal and other microclimates that have perennial cloudy skies.

jonr · January 2018

For people who heat with propane or natural gas, it does seem wasteful to not also generate electricity. But to date, I think that most find that there are practical and economic issues.

Advice request: Combined Heat and Power Solution for fully off-grid

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