This article is the first in a series outlining the technological and economic obstacles to the widespread use of solar power.
It all sounds so simple. Buy a solar gadget for your roof, tie it into the household electric system, get all the electricity you want, buy a rustic place in the mountains, live “off the grid,” and congratulate yourself for being independent.
Like most dreams, this one is based on delusion.
Alternatively, you might dream of having the solar gadget produce enough extra electricity that you could tie it to the grid and sell the excess to the utility company. This “on-the-grid” dream involves somewhat different delusions.
There is nothing as devastating to a beautiful dream as waking up to a brutal gang of facts. In this article and next month’s installment we’ll discuss the off-the-grid and on-the-grid scenarios in order.
In subsequent issues, we’ll look at the solar gadgets variously called photocells, photovoltaic (PV) cells, or solar batteries. Then we’ll discuss the ancillary systems required for any solar installation.
Let’s pretend, just for fun, that you could just buy the solar gadget–we’ll discuss that device in a future installment–and tie it into your wiring. OK, now how do you get electricity on a long, cold winter night? Clearly, the matter of storage rears its ugly head.
Sure, you can buy storage batteries, but something as small as the 30 pound lead-acid beast in your car is nowhere near adequate to produce your electricity for even one short night, let alone several nights and several cloudy days.
The batteries are neither small, nor free, nor everlasting. They are similar to car batteries, but different in manufacture and use. A car battery is rarely discharged more than a percent or so, unless somebody has left the lights on or the engine is balky about starting.
A battery used for backing up a PV system would be charged during sunny days but would be discharged during nighttime and cloudy days whenever somebody turned on a light or other electrical device. This battery would often be discharged to the point where it was almost dead.
On the other hand, while the starting current drawn from a car battery is often very high, the current drawn from the PV backup system battery would typically be much less.
For this purpose, there are “deep-discharge” batteries that are deliberately made to withstand such hard usage. The ones designed for PV systems are considerably larger than car batteries, both in physical size and in the amount of electrical energy they can store.
How many batteries to buy is a question with no easy answer, but the simple answer is “more is better.” Suppose your solar gadget can charge any number of batteries during one sunny day; it is still true that the number of sequential nights and cloudy days your system can handle is just a matter of how many batteries you have.
But don’t despair. Your PV system will be limited in how many batteries it can charge during a day, so it would be pointless to have a hundred batteries in the basement anyway. To quote an aphorism from E.F. Schumacher: Small is beautiful.
Economies of Scale
That brings us to a problem called “economies of scale.” Occasionally, you use a great deal of power, especially when the electric dryer and the oven are running while lights, the freezer, the refrigerator, the TV, and the computers are running simultaneously. At other times, you use very little power, as everything is turned off.
Measurements show a house will occasionally use as much as 15 kilowatts for short intervals, but in a neighborhood of eight to 10 non-air-conditioned houses supplied by a transformer, the power demand will not exceed about 3.5 kilowatts per house.
A substation handles the power for many distribution lines. The utility usually allocates about 2 kilowatts per household at this level.
From the standpoint of the power station, the utility needs to produce less than about 1.5 kilowatts per household. In other words, the local system of the off-grid user has to be designed to handle 10 times as much power as the power station would allocate to a single house.
Better Deal for Consumers
That is, the power station can take advantage of the diversity of demand, providing enough capacity to allocate each house about 1.5 kW. An off-grid system cannot do so–it must be ready to handle the maximum demand, which can easily be 10 times as high as the citywide average demand.
Hence, even if the per-kilowatt cost were the same for the off-grid homeowner and the utility, the homeowner’s equipment cost is guaranteed to be much higher than the utility’s equipment cost for the same household.
Howard C. Hayden, Ph.D. ([email protected]) is professor emeritus of physics at the University of Connecticut and adjunct professor at Colorado State University at Pueblo. He writes a monthly energy newsletter available for a $35 annual subscription at The Energy Advocate, PO Box 7595, Pueblo West, CO 81007.