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Two sources on energy, and using heat


I was reminded yesterday, after posting more on the cost-effectiveness of energy sources, to point out an interesting new book on the economics of energy. The book is Sustainable Energy With the Hot Air by David MacKay, a physics professor from Cambridge University. What's important about the book is that he pays hard attention to the numbers, and demonstrates that certain types of alternative energy are likely to never make sense, while others are more promising.

I only have a few faults to pick with the book, and he's not unaware of them. He decides to express energy in the odd unit of "kilowatt-hours per day" as he feels this will make numbers more manageable to the reader. Of course with time in the numerator and denominator, it's a bit strange to the scientist in me. (It's the same as about 42 watts.) In a world where we often see people say "kilowatt" when they mean "kilowatt-hour" I suppose one deserves credit for using a correct, if strange unit.

My real quibble is over his decision to measure energy usage at the tank, so that an electric car's energy usage is measured in the battery, while a gasoline car is measured in the fuel tank. Today we burn fuel to make electricity, and so electric cars actually consume 3 times the energy they put in the batteries. That's a big factor. MacKay argues that since future energy sources (such as solar) might generate electricity without burning fuel, that this is a fair way to look at it. This is indeed possible but I think it is necessary to look at it both ways -- how efficient the vehicles are today (and will be if we still generate electricity from heat) and how they might be in the future. Generating electricity from heat does complicate the math of energy in ways that people can't agree on, so I understand his temptation.

Yesterday I was also pointed out to a solar power site called SolarBuzz. This is a pro-solar-panel site, and is rare in that it seems to do its math right. I haven't looked at all the numbers, and I am surprised wthat with the numbers they show that they are such boosters. Their charts of payback times all focus on power costs from 20 to 50 cents/kwh. Those costs are found in Europe, and in the tiers of California, but the U.S. national average is closer to 10 cents, where there is no payback. They also use 5% for their interest rate, a low rate that is only found in strange economic times such as these -- but justifiable in a chart today.

Pure heating is highly wasteful

Another area I plan to investigate more relates to the heating of buildings and water. After cars, this is one of the other single areas of high energy inefficiency.

How inefficient that is is remarkable. A general assertion I have seen often in modern building design is that we know today how to build new buildings that need no conventional heating or cooling over the entire climate range of the 48 U.S. states. When told this, I have asked how it is that we don't build such buildings. The answer is that such building style adds to the capital cost of the building, while conventional heating and cooling come out of the operating cost. In commercial buildings, the builder and owner who pay the capital costs are different from the tenants who pay the utility costs, and nobody has the guts to borrow more money to build a more expensive building to save the tenants money down the road. If you can borrow more money, build a fancier building for more profit today -- this seems to be the rule.

The same is true in residential housing. Home buyers shop in a price bracket. They don't say, "I want to see $300,000 homes with a $2000/year heating/cooling bill, and also $330,000 homes with no heating/cooling bill." They follow the rule of buying the most home that your bank says you can afford. (A rule which got many people and banks into trouble.)

We can't retrofit all our existing buildings to full Passive solar building design but our standard method of using a furnace that burns fuel and sends the heat into the house is quite wasteful. We call our furnaces 90% efficient, because 90% of the heat they generate goes into the house, and 10% up the chimney. But the truth is that in the world of energy physics, areas of high temperature next to colder heat sinks represent more than just heat. Big heat differences can be turned into far more useful energy like electricity.

Today I heat my house by burning natural gas, while a remote natural gas plant runs generators that send electricity to my house. Even though big generators are more efficient than smaller local generators, those big generators throw away 60% (it varies) of the energy they generate as waste heat. While a smaller generator might throw away 70%, the heat need not be wasted if it is generated locally. It turns out that having a less efficient onsite generator which produces electricity and pumps the waste heat into home heat and hot water is a fair bit more efficient, based on reports I have seen, than our current setup.

Typical home natural gas generators have downsides. They are not that efficient, and they make noise, and they need some maintenance. Newer microturbine technology holds more promise -- generators almost as efficient as the big turbines at the power plant, with little of the heat wasted. They are a little large for a house, but this approach makes a lot of sense for both collections of houses, and for all larger buildings.

This is particularly true with grid intertie. The generator can make electricity when heat is needed (for hot water or building heat) and feed that into the grid. So the generator would run not when there is local electrical demand, but when there is local heat demand. It might also run when there is special electrical demand as long as there is somewhere to put the heat. Of course it would also be a fine backup generator for power failures, and a peak load generator for the power grid in general.

Of course peak loads come in the summer, and in the summer such generators would need to divert their heat elsewhere. Once the hot water tank was at full temperature, the heat would have to go out the chimney or into a swimming pool. Or the generator would just not run as much in summer except to meet peak demand. I would be curious to see if having a mechanical linkage to the air conditioning compressor might make the air conditioner efficient enough that it is better to do that than to run it on an electric motor using power from the power plant. This of course would be great for reducing peak loads.

I am also interested in claims that show heat pumps as being more efficient than ordinary burner heaters. Heat pumps, in winter, gather heat from the air, the ground or swimming pools and transfer it into the house and hot water. Some figures show it is more efficient to burn fuel in a power plant, throwing away the waste heat, in order to make electricity to run heat pumps at houses. In that case it should be much more efficient to burn fuel to run a heat pump's compressor locally, and also use the waste heat.

(In some cities and large campuses, the waste heat from power plants is piped through steam tunnels to heat buildings, so it is not wasted.)

Of course, solar heating for water is even more fuel-efficient. Because solar water heating needs the backup of a gas water heater in most climates, it is not so commonly used due to capital costs. Capital costs might also stand in the way of more microturbine and heat pump installations, no matter how fuel efficient they are.


As you pointed out above with hybrids you lose a fair amount of energy just converting it from gasoline into electric power. Additionally, the bigger the battery you put in the car to extend its range in the electric mode, the heavier it is to move the car around. In San Francisco, some of the bus lines are powered by overhead electric wires. I was wondering about extending that overhead grid and using it to power bumper car style cars (cars powered off overhead wires). It seems like that would remove a lot of weight from the cars, you wouldn't need to use electricity to push either batteries or gasoline around, so you could probably get by with a smaller engine. Also you would be drawing power directly off the grid using what ever local clean power is cheapest (hydro, solar, wind, geothermal or nukes). The bumper cars at Great America were pretty light, but I have no idea how fast they went.

Cars may need two wires (like a trolley bus) which is more complex, and has many other issues. Plus you can only go where the wires are -- or you need batteries or an engine anyway. You get wires ruining the look of the streets, unless one can design a safe ground based power system, which is hard to do to work in the rain and deliver all that current.

Lots of power means high voltage, and fire risks.

I read most but not all of MacKay's book, missing the section on cars. Excellent point about electric car's energy uses. MacKay is very concerned about "carbon footprint." A critical error not to count the fossil fuel use to make electricity for an electric car.

Where I live, electricity is about 80% from coal, 10% natural gas, 4% hydro, others minimal. Although the utilities are building mostly new gas turbine generation plants, gas is still a pretty expensive way to make electricity. Residential microturbines, especially with a mechanical linkage to the air conditioner are intriguing. I suspect they won't be competitive in the current market, because the turbines wouldn't have enough operating time to justify the capital expense. Interesting question how the economics would change with increasing fossil fuel prices. Free market resource consumption economy prevailing, I bet centralized coal fired electrical generation blocks out residential gas microturbines for decades to come.

Although it doesn't seem politically viable, IMHO the logical public policy approach is an increasing coal extraction task at the mine. You pretty much nailed the issue with subsidizing residential PV.

Great post, deserves wide reading.

I had an argument with the salesperson when i decided to buy some more contemporary radiators, for an extension to my house. He tried to convince me that they were more efficient than standard radiators!!

Demonstrates that certain types of alternative energy are likely to never make sense, while others are more promising.

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