12 June 2008

Solar Assisted Power Generation

Most of the focus in renewable energy research is on pure renewable energy technologies, like photovoltaics, solar thermal power plants, windmills and tidal power. But there is one approach that I haven't seen discussed that seems to make sense, hybrid solar-conventional power generation.

Fossil fuel and nuclear power plants are glorified tea pots. They boil water to create steam to turn turbines to generate electricity.

The energy involved in bringing liquid water to the 100 degrees Celsius at which is turns to steam is well defined. The amount of heat required is equal to the number of degrees of temperature increase required times the volume of water to be heated.

The hotter the water you start with is, the less energy it takes to turn water to steam, and hence, to generate power. It takes half the amount of energy required to heat water from a typical tap/river/lake water temperature of 20 degrees Celsius (68 degrees Fahrenheit) to 100 degrees, to do so when you start with water at 60 degree Celsius (140 degrees Fahrenheit).

Solar energy is relatively "low quality". What makes a solar thermal power plant hard to build is that you must concentrate diffuse sunlight into a single place to generate temperatures high enough to generate electricity. The lower the temperature you need, the easier it is to use solar power to achieve that temperature. Even a simple greenhouse made from plastic wrap, or a water bottle painted with black paint, can produce significant temperature gains, even though neither would ever boil more than trace amounts of water.

As a result, the main commercial use of solar power now is heating water to sub-boiling temperature, either providing a boost to a back up conventional water heater (see also here) in a building by pre-heating the water, or heating swimming pools.

But if you don't insist upon a pure solar system, the need to concentrate solar power is less great. By pre-heating water going into a conventional natural gas, coal or nuclear fueled power plant to 140 degrees Fahrenheit, using a far simpler than usual for electricity generating solar power system, you can effectively make your system 50% solar. Even pre-heating water to a mere 104 degrees Fahrenheit from room temperature should produce a 25% fuel savings.

Some conventional power plants use the conceptually similar idea of using waste heat from boilers (also here) to pre-heat water going into the boilers (or for other co-generation ends). But this approach is inherently limited by the fact that no new energy comes into the system from any source other than the boiler, combined with the fact all thermal systems are less than 100% efficient (this is the second law of thermodynamics). While reusing waste heat is also a good idea (and would use many similar technologies to the ones that I discuss here), it is essentially a way to make the plant more efficient, rather than providing an actually increase in the amount of available energy to drive the plant.

A hybrid solar-conventional power plant has other virtues as well.

As I've noted before, the beauty of solar power for electricity generation is that demand for air conditioning is closely tied to available solar power generating capacity. Hot days tend to be sunny ones with intense sunlight for reasons that are not coincidental. And, peak annual electricity demand is largely a product of air conditioning demand.

Thus, a hybrid solar-convention power plant is most efficient (and hence provides power at the lowest cost per kilowatt hour) when demand is greatest. In contrast, many electrical utilities now use natural gas, the most expensive commonly used fuel to generate electricity, to respond to peak summer demand.

But unlike a pure solar power plant, a hybrid plant doesn't have to devise infrastructure intensive ways to store peak solar power for nights and cloudy days, or have a back up power plant. At these lower demand time periods, baseline power is naturally provided by conventional fuels.

The environmental benefits of a grid powered by hybrid power plants that use 20% less fuel because of solar pre-heating of water heated in the plant are the same as the environmental benefits of having a grid in which 20% of the power plants are exclusively solar, and 80% are exclusively conventional. But hybrid power plants use 20% less conventional fuels than fully conventional power plants, with the savings concentrated on hot sunny days where there are savings of 50% or more from solar assistance, should be far less technologically challenging and somewhat less expensive to build than building enough exclusively solar power plants to serve 20% of the power grid. Indeed, since existing power plants could probably be upgraded to have solar assisted pre-heating of water boiled at the plant, it would probably be possible to avoid even the cost of building entire new power plants from scratch.

Less fuel also means fewer air pollution emissions. Burning 20% less fuel is a much easier way to reduce emissions by 20% than scrubbers and the like. And, unlike traditional air pollution control measures, that do essentially nothing to reduce global warming inducing CO2 emissions, burning less fuel does reduce the CO2 based carbon footprint of a conventional power plant.

Even a modest 10% fuel savings from solar assists, which would require pre-heating to a mere 104 degrees Fahrenheit for 50% of overall power generation, with a less than perfectly efficient system, would make solar power a larger share of the overall electrical power supply than any state in the United States receives from non-hydropower renewables today.

I'm not an engineer. I don't even play one on TV. This is just a back of napkin idea from an educated layman. But the concept of solar assisted power generation does seem to me to be an underdeveloped concept.


Kevin Dickson said...

Wait a sec, the water temp coming out of the turbine is still too hot for non-concentrating solar panels to help out with. These steam turbines recycle most of the water.

And most of the heat must be supplied at over 100C to overcome the latent heat of vaporization of water, or the phase change to steam. Six times more energy goes into the phase change than is required to bring the water from 60C to 100C.

In short, most of the heat needed is at a higher temperature than non-concentrating solar can handle.

Andrew Oh-Willeke said...

Good point. I knew there were phasechange energy costs but had forgotten just how big they were. Still, while the percentages are smaller, a 3-6% savings seems possible.