May 31, 2009

Has it really been two weeks since the last post? Yes, it has.

The issue under discussion then and now is how does one produce enough energy from renewable sources in one's own residence to dispense with utility supplied power while maintaining precisely the same lifestyle?

You'll be hard put to do it with solar panels. Very hard put. Do the math, as they say. You'll be lucky to get more than 300 watts on average per square meter of surface area from PV over the course of the day. Even a big installation approaching 50 square meters will only get you to about half of what you're probably taking from the grid.

Wind, sad to say, is plainly impractical for most homeowners. Those little one kilowatt jobs you can stick on your roof or install in your back yard crank out a kilowatt only at maximum output which happens at the maximum wind speed for which they are rated which is something less than a gale but more than a stiff breeze. A giant 100 kilowatt turbine might meet your needs if you could get it permitted, but then again it might not. Conventional turbines like to reside in wide open spaces, not in suburban neighborhoods. All those obstructions on the ground known as houses disrupt the wind flow and rob the turbine of power. And may end up destroying it because turbines function poorly in the presence of turbulence.

An interesting alternative to wind and PV is a technology known as concentrating solar. No established firm is making products for the home owner, but a company calling itself Infinia is gearing up for production.

Concentrating solar technology has a number of embodiments, but all of the stuff I've seen intended for the residential market fits into one subcategory, the solar dish Stirling generator. This is configured as follows.

The system consists of a solar collector, a heat exchanger, a Stirling cycle engine, and an electrical generator. It puts out 60 cycle 115 volt AC just like a diesel powered electrical generator.

The solar collector is comprised of a number of mirrors arranged in a dish arrangement kind of like a large satellite dish. The mirrors themselves are flat, and the collector is faceted with some open areas to reduce wind loading. Sunlight is focused on black heat exchanger in the center of the dish.

The heat exchanger generally contains molten salts which are circulated over another heat exchanger on the Stirling cycle engine. This transfers heat to the working fluid of the Stirling which is generally helium.

A Stirling engine, named after its inventor, one Robert Stirling, is a closed cycle hot air engine (helium is used in present examples because it is more thermally conductive than air). Stirling developed the engine as an alternative to the steam engine, his theory being that it would be safer, which it was and is, but an added benefit is efficiency. The most efficient modern Stirlings approach 50% conversion efficiency.

The total system efficiency of a dish Stirling concentrating solar system is only about half that but it's still more than twice that of a PV array. Wind generators can achieve considerably higher conversion efficiencies but there the resource is inconstant and unpredictable.

A dish Stirling occupies about the same footprint as a wind turbine of comparable output, but siting constraints are far less stringent and the height of the concentrator array is a fraction of that of typical turbine mounting tower. Altogether it looks to be a promising if neglected technology.

So why hasn't concentrating solar happened? Both the collector assemblies and the Stirlings have been very expensive. Part of that has to do with economies of scale but to date no one has demonstrated low cost manufacturing techniques for either the mirror arrays or the engines.

Another factor is intermittency. When the sun is out a dish Stirling will produce a fairly reliable output but it can't operate at night in the absence of thermal heat. Unless the some of the heat collected is conserved in a thermal storage medium.

Thermal storage medium can be molten salts, fire bricks, cast iron, or even concrete. Normally thermal storage is pretty massive but it can be cost effective in and of itself. The problem is you need approximately twice the collector capacity as otherwise because you have to heat up the storage medium as well as running the generator.

Right now there is a big push for concentrating solar for public utilities because it can be used to provide baseline power, unlike wind, but investment has slowed in this area and so far there's only one real commercial installation in existence. Whether it can compete effectively against wind remains to be seen. Residential concentrating solar seems further off, but it's likely to work in a greater number of installations than wind power, if the price can be made competitive.