[energy] Conservation in the Home
In the US, 22% of energy is consumed by the residential sector, and another 18% by commercial buildings. This is also the area where most of us have direct experience of what goes on. I'll start with what I know best, and write what little I've managed to learn about the other sectors later.
Nationally, almost half of energy consumption in the home is for space heating. You can see from the chart that electrical appliances and hot water follow. Since neither pika nor my apartment uses much air conditioning, I'm going to put that off until I write about commercial buildings. pika roughly follows the national pattern. Per capita (assuming 30 residents, a number which goes up and down), we're looking at 9.3 GJ/year on heating, 6.7 GJ/year on electricity, and 5.9 on hot water.1 If you read the little numbers on the chart, you can work out that pikans use about as much hot water as national average (we do shower), but rather less on heating and electricity (more people in the same space).
Which is all very well, except that we're interested in carbon, not energy. So we need figures on the carbon content of our two forms of energy. This is easy for natural gas; the value is simply 50.6 grams CO2 per MJ. An equivalent number for electricity requires knowing something about the portfolio of fuels used to generate electricity in Massachusetts, and typical plant efficiencies.2,3 The value comes to about 600 grams CO2 per kWh, or 167 g/MJ. Converting thermal energy to electricity is limited by entropy, and real processes don't come close to theoretical efficiency. So don't heat things with electricity, and prefer high-entropy energy when it will do the job.
The upshot of all this is that while most of the energy consumed in the home goes to space heat, most of the carbon goes to electric devices. I've got some cheap current sensors from digikey, and tomorrow another pikan and I are going to try to assemble a logging meter to measure where all that electricity is going. Preliminary guesses include lights in public areas that are always on, refrigerators, the sterilizer (yes, electric heat), and the old desktop machines in the athena cluster. We'll be looking at installing timers or motion detectors to control lights and sterilizer. Newer refrigerators are dramatically more efficient than those sold 10 or 20 years ago, and continue to improve, mostly under regulatory pressure. I suspect pika both has more public computers than are used, and can save energy by configuring better power-save and shutdown behavior. I'll write more about this when we have data, and a plan.
I'm going to save space heat for another post, so that just leaves hot water. We're installing some meters at pika for that also, mostly to answer how much hot water we use washing dishes. The steps for saving water in showers are pretty obvious. Low flow shower heads are pretty much the norm; that typically means 2.5 gallons per minute. I'll eventually get around to trying out one of the newer models in the 1--1.2 GPM range.. Turning off the water while lathering up helps; some of these shower heads provide a valve for the purpose.* So conservation cuts water use by half or more, and then we turn to fuel substitution. pika is planning to install a system of evacuated tube collectors, and we expect that pika can provide 70% of current demand year round with less than 1 m2 of collector per person.
A southerly surface receives about 15 MJ/day/m2 of solar energy on a clear day even in winter. Evacuated tube collectors, heating water to 50C, can capture around 9 MJ/day/m2, before losses in other system components. A pump will circulate antifreeze through rooftop collectors, then to a heat exchanger in the basement, where hot water is stored until needed. The collectors are manifolds in which the stream of antifreeze meets a set of heat pipes. Within these heat pipes, a working fluid is vaporized by heat from the pipe walls, rises until it comes into thermal contact with the antifreeze, and then condenses and falls back down the pipe. The pipe walls are joined to a fin which is the actual absorber surface, and which is treated to have high absorptivity in the solar spectrum, and low emissivity in the long infrared. This assembly of heat pipe and absorber is surrounded by the evacuated tubes which give the system its name---two concentric glass cylinders, closed at either end, with perhaps a tenth of an atmosphere of pressure between them. On a sunny day, output temperatures can easily exceed 100C, although if you want steam, there are probably better ways to do that.
*edit: Apparently low-flow showerheads are universally despised. I'm pretty sure that the best and worst showers I've used were all rated at the same nominal flow rate, but now I'm curious. I'm plotting further experiments.
1These numbers come from pika's gas and electric bills. I've assumed that the gas used in warm months goes entirely to heating water, but obviously some of it goes to the stove. I assume that pika uses no more hot water in cold months, so all the additional gas is for space heat.
2Greenhouse Gas Reduction with Renewables, Electric Power Research Institute, chapter 5
3Net Generation by State by Type of Producer by Energy Source, United States Energy Information Administration