Saturday, 11 February 2012

Microeconomics of solar power

As we look at our power consumption and think about ways of shifting it from daytime, when we lose the opportunity to sell our solar power, to night time, when they are trying to give it away, three basic approaches spring to mind: Design, technology and habits.

We also need to think about comparative power consumption. Lighting began as the major user of domestic electricity, and indeed the Japanese word for electricity, denki, is synonymous with the word for light. But today it represents a tiny fraction of our power use. Our electricity display panel shows our consumption down to the nearest 0.1 kW, or 100 watts, and switching lights on or off has never made any difference to this. Leaving every light in the house on would use perhaps 200 Watts, and require a great deal of running around, inside and outside, as several of the lights come on automatically. We seem to be using very roughly 20kWh per day, over four times this hypothetical maximum use. Of course we don't use any of the lights all the time; neither during the day time, when we're out nor when we're asleep.

The sensors on the lights seem like a good idea as they can't be left on. Someone was complaining that they stayed on a long time after being activated, about a minute, and it would be good to be able to regulate this. These lights use about 6 Watts, so if they're on for a minute, that's 0.1 Watt hours, or 0.0001 kWh. Boiling the kettle uses around 2kW, two thousand watts. Leaving the kettle on for an extra second is equivalent to leaving one of the lights on for five minutes. Watching the pot boil, and not  putting too much water in it makes much more sense than worrying about putting lights on. In fact, in terms of energy usage, I'm not sure that the decision to use sensors was sensible. It certainly makes sense in terms of light switches, or their absence, but that is another story.

In rough orders of magnitude, heating appliances use a hundred times more electricity than lighting appliances.

A lot of technology exists that can shift power consumption into the night time. For example our washing machine has a timer so we can set it to finish the cycle by 7am, and use the night-time electricity. The washing machine is using hot water from the boiler, and electric motors use less power than heat, but more than light. The washing machine has a heat pump to help with the drying cycle, which uses more power than the motor, but not as much as a heater. I'll come back to that in a moment.

The dishwasher, on the other hand, has no timer, so it relies on us remembering to set it off after 11 at night, or first thing in the morning when our electricity is cheaper. Also, the dishwasher heats cold water, rather than using hot water from our boiler. We were advised that this was wise, although that was before we realised that the adviser did not know about pipe insulation, and in fact the manual for the dishwasher advises using domestic hot water to save energy.  

The rice cooker has a timer, and we have been routinely washing rice at night and setting it to be ready for breakfast, so we are using electricity at the cheap rates.

The boiler, AKA Eco Cute, has a heat pump, which uses the power of a compressor to bring heat from the atmosphere. This uses a lot less energy than it would to directly heat the water with a heating element. The ratio of heat out to energy in is known as the COP or coefficient of performance. If the heat pump has a COP of 5, then it will get 5 units of heat energy out for each 1 unit of electrical energy. I need to look into what exactly the COP is, and how it varies with temperature, but if we were to take it as 5, and compare the dishwasher using hot water made in the Eco Cute at night time with hot water made from cold water in the middle of the day, then the former would use five times more electricity at five times the rate, and so would be twenty-five times more expensive.

If the design is right, and we use appropriate technology, we don't need to worry so much about changing our habits.

If we make breakfast before 7am, this will save us money. Easy on a weekday. Can be tough on weekends. Putting the oven and kettle on, and cooking with frying pans uses a few kilowatt hours. In fact before the breaker was boosted from 60 amps to 50 amps, the breaker would trip if the kettle and oven were both on while the eco cute was was still boiling water, and the washing machine or dishwasher were on. So we were using perhaps 5 kW to make breakfast. We weren't using all those kilowatts for an hour, but over a week, that adds up.

Another thing that would make a small difference is an electric thermos flask, with a timer to boil water before 7am, rather than a kettle using live electricity.

I'm sure, in an ideal word, the fridge would do it's cooling when the electricity is cheap and abundant, whereas now it comes on when it feels like a chill. This is more likely to happen in the middle of the day when it's hotter inside rather than the middle of the night. The fridge seems to use a couple of hundred watts for its heat pump.

It would be nice to have a power logger on each appliance so that we can see when it's using electricity. I've started tracking the hourly electrical consumption and production, so we should be able to infer some of the major users.

Apparently Eco Cutes, which have been sold largely to utilise night time electricity, have been so successfully sold in Hokkaido, the northenmost, coldest island of Japan, that night demand is now stretching the generating capacity and they're considering building more power stations.