The Fuel Cell For Home

The big win is having the waste heat available at the house.

I suspect it's a question of what you do with the waste heat. Combined Heat and Power (CHP) stations are very efficient, but you need a way to distribute heat around the neighbourhood, which is expensive. A home fuel cell could generate electricity efficiently, plus put the waste heat right where you need it for space heating or hot water.

A potential factor is improved efficiency due to not relying on an electricity transmission and distribution grid, and generating the electricity where it is needed...

1. Electricity grids have to account for a loss factor due to the accumulated resistance of the wires between the power station and where the electricity is actually used. This power is lost as heat radiating from the transmission wires. That said, I'm unfamiliar with gas transmission networks so I would assume there is some degree of potential energy loss through gas leaking from pipelines, etc. I expect it is lower than electricity grids however?

2. If the power generation is decentralised, I would assume that less would be spent on electricity substations, although that might be countered with a greater spend on maintaining the gas distribution network. I'm not an engineer, but I get the feeling that perhaps maintaining the pressure of gas to balance fluctuating demand is easier than maintaining the voltage/frequency of AC in a grid?

2.

FCs don't operate via the carnot cycle so in theory can be 90+% efficient in electricity generation. in practice, 35-50% electrical efficiency is possible (although this isn't a ceiling, just a current tech benchmark). coal fired is 25-30% and gas fired 30-40% ish - i think the carnot efficiency maxes out somwhere 40-50% so no matter what you do (and we've been doing this for 100 years now) you aren't going to get 60+% electrical efficiency from burning stuff to drive a turbine. the FC remainder is thermal and in the situation of a combined heat and power unit, this is very useful as a lot of energy is just used for space heating - why waste high grade electrons if you've got some waste heat kicking around anyway. from the users perspective, 50% electrical and 50% thermal is getting close to an ideal 'efficiency' for actual use. i used to work on these systems in the late 90s and there's nothing new here. the dream of having a complicated steam reforming unit coupled to a Faraday device that is expected to run like a fridge with zero user maintenance for 3-5 years at a time is tremendously difficult.

This system is integrated with domestic heating requirements. If you run the system enough to heat the house and provide hot water, then any electricity it makes is effectively at 100% efficiency. This can easily make a lot of sense in cold places.

Commercial natural gas electrical generation facilities are about 35% efficient. It is probably inappropriate to consider transmission losses since natural gas power plants tend to be located in areas of high consumption, but that 35% only goes down on the way to the end user. Remember, it doesn't have to beat the utilities for it to make sense.

Consider a house with a $200/month natural gas bill for heating. At $1/therm that is 200 therms of gas. Let's just guess at around 10% efficient for these cells, 220 therms of gas would give us our 200 therms of heat plus 20 therms of electricity which we convert to kilowatt hours, about 600 kilowatt hours. At $0.12 per kwHr that gets us about $72 of electricity for $20 in gas. Let's just say for every $3 we currently spend on gas we can get $1 of 100% efficiently generated electricity (1/3 the carbon footprint).

At 37°N, in December it would cover half of my electricity for a free standing house (but remember I made up their efficiency. At 20% it would cover all of my electricity. Googling about shows a record high around 60% and some press releases around 50%. I have no idea what these are, but it is probably several times the 10% number I used.)

During summer when I'm using nearly twice the electricity my gas bill is so close to zero as to not matter.

Zigurd, I doubt the "direct" 'gas to electricity' claim in this press release.

Near the bottom the press release admits the conversion /isn't/ actually "direct", stating:

  If the fuel cell heater is connected to the gas network, 
  a reformer initially converts the natural gas into a 
  hydrogen-rich gas.

Ah-ha: A "reformer": That would be a 'steam reformer[1]'. Quoting [1], here's what a 'steam reformer' does:

  At high temperatures (700 – 1100 °C) and in the presence
  of a metal-based catalyst (nickel), steam reacts with 
  methane to yield carbon monoxide and hydrogen. These two 
  reactions are reversible in nature.

    CH4 + H2O ⇌ CO + 3 H2

So there it is, the 'hydrogen-rich gas' is syngas [3], one mole carbon monoxide plus 3 moles hydrogen.

The syngas can either be burned directly (producing CO2 and heat of course), or continuing per [1], separated from the H2, then combined with more water:

  Additional hydrogen can be recovered by a lower-
  temperature gas-shift reaction with the carbon monoxide 
  produced. The reaction is summarized by:

    CO + H2O ⇌ CO2 + H2

But the article uses just one word, 'afterburner' which tells me they're burning away the carbon monoxide:

  The researchers were particularly responsible for the 
  construction of the prototype, the design of the overall 
  system, the design of the ceramic components and the 
  development of the reformer and the afterburner.

Not clear is whether the CO goes /through/ the ceramic SOFC or is separated so it can /bypass/ the SOFC then get remixed with some H2 at the 'afterburner'. No mention of a 'separator', so perhaps the CO does go straight through the SOFC along with the H2 without ruining the SOFC.

In any case, we see this CHP plant with its SOFC requires water in, makes carbon monoxide in the process, which it 'afterburn[s]', certainly putting out CO2 [the Carbon from the methane can't just disappear, would you have that as carbon monoxide or carbon dioxide?], and needs some heat to run the steam reformer. There goes some of your thermal efficiency!

[2] has a bit more on home CHP.

So long story short, there's no free lunch here:

* It can't hit 100% thermal efficiency

* It releases CO2

* Its intermediate stage passes H2 through, and CO either through or around the SOFC, then burns off the CO to CO2

* It needs water as well as natural gas to work

Presuming the Bloom Boxes also use SOFCs and methane, the operating chemistry would be the similar at the inputs and outputs: Natural Gas and Water in; Heat, CO2 and Electricity out.

There may be an advance in materials science if the SOFC accepts syngas ( CO + 3 H2 ) directly without bypass of the CO and without cell degradation from the CO, but the press release says nothing specific about, or suggestive of that.

edits for dialog form, clarity, corrections, 'afterburner' clean up the CO in the syngas to CO2, no water out, oh it says press release right at the top, no wonder it reads like one

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[1] https://en.wikipedia.org/wiki/Steam_reforming

[2] https://en.wikipedia.org/wiki/Home_fuel_cell

[3] https://en.wikipedia.org/wiki/Syngas

I've been keeping a casual eye on them -- they seem to be selling to big customers, but at $750k apiece. They're also the size of a bike shed, and generate 100 kW.

This is the size of a stack of CDs, and generates 1 kW. (Which is enough for lights, tv, etc, but generally not the stove.)