Will there be a role for combined heat and power in an all-electric future?

Question

Beneficial electrification

From the National Resources Defense Council:

“Beneficial electrification,” a new catchphrase in the energy world, refers to the growing recognition that using clean electricity to keep our homes and businesses running is cheaper, greener, and a smarter way to meet our energy needs.

Essentially, using electricity to power end-uses that used to require fossil fuels is a straightforward way to reduce pollution and GHG emissions, if the electricity is generated by wind, solar, or hydro plants. With the 2C deadline approaching in less than 20 years, it seems obvious that beneficial electrification will be one important strategy to reduce emissions and avoid climate catastrophe.

Combined heat and power (CHP)

However, another emissions-reducing strategy is CHP, which uses a single system to generate both heat and electricity. Generally this involves a natural gas- or diesel-burning electric generator which uses the otherwise "waste heat" for things like water heating, space heating, or process heat -- things which would normally have their own separate heat source.

By combining the systems, total efficiency can be boosted from 45% to as high as 80% (source), representing a huge opportunity to reduce emissions.

The question

These two separate strategies for reducing emissions would seem to be at odds -- one aims to stop burning any fuel, the other improves efficiency by changing where the fuel is burned. But is there a role for CHP in an all-electric (or even more-electric) future? Or should research and incentive funding for CHP systems instead be channeled to electrification?

Answer 1

CHP combines a thermal power station with a system for distributing spare heat to customers near the power plant. Thermal power stations driven by fossil fuels have no future, but there is likely to be continuing demand for thermal power stations driven by bio-fuels or hydrogen, as these technologies provide an affordable means of storing energy until it is needed. As long as there are thermal power stations in use, there will be some demand for CHP. Less than now, because the number of thermal power stations is going to be massively decreased as they are replaced by wind and solar power. The spread of heat pump technology makes the use of CHP for domestic heating less attractive. With heat pumps it becomes economical to heat a house with electricity, and this avoids the overhead of plumbing in to a local hot water network. Industrial uses of CHP waste heat are likely to continue (e.g. heating greenhouses).

Answer 2

A combined-heat-and-power system can use the waste heat of a solid-oxide-fuel-cell to steam hydrogen out of natural-gas or out of propane. Then the hydrogen easily produces electricity using the SOFC. However, the waste heat can also easily go into a boiler that is connected to steam radiators for primary building heat.

Here is a supporting link for a residential SOFC:

https://www.prnewswire.com/news-releases/peoples-gas-completes-installation-of-the-first-watt-residential-fuel-cell-in-western-pennsylvania-300723773.html

But a commercial powerplant could produce hydrogen from reforming of most any liquid or gas fossil-fuel using the waste heat of a SOFC, run the hydrogen through the SOFC to produce electricity, possibly produce more hydrogen than is needed and therefor gain marketable hydrogen, but finally capture the carbon-dioxide from the process to produce plastic.

See, the power companies have expertise in building smokestacks while the chemical companies have expertise in building elaborate systems for chemical processes. If the powerplants were built like chemical plants then it might be possible to have zero-carbon-release powerplants.

Finally, a hydrogen-fuel-cell that uses its waste heat can have efficiencies of about 85%. A gas-turbine that uses its waste heat can have efficiencies of about 65%.

Answer 3

The waste heat from a fossil fuel steam plant is fairly low grade -- typically at the temperature of wet steam (~100 C) The problem has typically been how to get this heat from where it's made to where it's needed.

A gas turbine can do better than this, using it's waste heat to power a secondary medium temperature steam cycle. This is not what's usually meant by co-gen. The waste heat is still fairly low grade.

Story: I live 3 miles from Capital Power's Genesee Power Plant. It's one of hte better coal plants averaging close to 40% efficiency over all for the 3 units built over the last 35 years.

At one point they were looking at making a pipeline to take hot waste water to downtown Edmonton for use in heating buildings there. A 100 km long 4 foot diameter pipeline wasn't in the cards.

I did the figures, and the waste heat would do a nice job on about 2 square miles of heated greenhouse.

One company was going to use it to raise tilapia, a sub tropical food fish that grows well in tank culture. Capital Power was interested and cooperative. The fish company figured there wasn't a good supply of local people willing to work at low wages in the immediate vicinity.

Answer 4

But is there a role for CHP in an all-electric (or even more-electric) future?

Waste heats will be used. One that's already being used is heat from wastewater (converted using heat pumps to higher temperature). Also data center heat is increasingly being used to supply district heating networks with heat.

However, I suspect CHP usage amounts will reduce, being replaced by different kinds of waste heats. The main reason is that previously fuels were burned in baseload power plants that had combined cycle turbines and the waste heat was used to supply district heating networks. Those baseload power plants had capacity factors close to unity. They did somewhat follow the load on the electricity grid, but they were used more than standing still.

Baseload using burnable fuels can't compete in the modern environment. Wind and solar power will account for the vast majority of power production. The daily fluctuations in solar power and the varying electricity usage during day and night will be evened out by massive grid-connected batteries. Whatever hydropower we have will be used to follow the load in the electrical grid. Burning fuel simply can't compete with these better ways of producing and storing electricity.

However, the big question is, can we construct enough hydropower? Nearly all "free" hydropower where energy is supplied to us via rainfall is already being used. The only opportunities we have remaining are pumped storage hydro. Pumped storage hydro works best in mountainous regions, but not everyone lives close to mountains, so there's another option, construct hydrogen-burning power plants closer to electricity users, working at efficiency of 50% if producing only electricity. The rest 50% is waste heat, and it can be reused in CHP. The hydrogen would obviously be produced using electrolysis when renewable electricity is massively and cheaply available, and the waste heat from electrolysis would supply the district heating networks with heat when electricity is cheap.

So when electricity is cheap, electrolysis provides us the waste heat. When electricity is expensive, hydrogen CHP provides the waste heat. When electricity is mid-priced, neither can work, but fortunately large-scale heat storage is easy by storing heated water in massive caves.

So what determines the fate of CHP is whether we can have enough pumped storage hydropower to supply all areas with adjustable electricity production. If yes, then hydrogen CHP can't compete with its miserable efficiency and expensive fuel. If no, then hydrogen CHP will have to be used, but the amount of heat produced by CHP will decrease, since only 15% of our electricity would come from adjustable power plants, and out of that probably at least half will be hydropower, so at most 7.5% would be the share of CHP.

So CHP might have a future, but used only intermittently, with the rest of heat being reuse of other waste heats (electrolysis, wastewater, data centers) and heat storage. And even that's a "might" and not "will", since it is indeed possible that pumped storage hydro can solve all our electricity storage needs, in which case hydrogen would be still produced (for steelmaking and fertilizers and other chemical industries), but never converted back to electricity in CHP plants.

In any case, the only fuel CHP plants will use in the future would be hydrogen, simply because it's the most reasonable fuel that doesn't produce any carbon dioxide. Mobile installations (ships, airplanes etc) might see a benefit from ammonia as hydrogen carrier, but fixed installations would use compressed hydrogen stored underground and delivered in a pressurized gas network.