During the 1970's, Solar re-powering was reviewed by the Department of Energy.
Coal power plants were reviewed for conversion. But today, the technology for
solar energy has much advanced; There are several methods of converting.

Charles Bensinger - past president of the New Mexico Solar Energy Association,
with a grant from the San Francisco based Energy Foundation
studied converting a coal power plant on Navajo Land in the
Four Corners Region....not much work has been done since then.

Solar collectors (of various types, flat plates, vacuum tubes, troughs, dishes, and so on)
make hot heat transfer fluids, basically hot cooking oil.
Then these hot oils are pumped to the power plant.
Most coal power plants have a 'pre-boiler' that preheats water to
pressurized water at the 'triple point'. The triple point is water
that is a solid, liquid, and gas at the same time.
Then the pressurized water is pumped to the firebox
where coal is burned. An exchanger, which
transfers the heat from the hot solar produced oil, then
pre-heats the water for the preboiler.
Thus the pre-boiler does not burn as much fuel.

Another method of relieving the heat load, displacing coal with solar
is old fashion hot air. Solar collectors again make hot, compressed
and high velocity hot air, putting them into underground
heavily insulated pipes. The power plant now instead has hot air blown at it.
Instead of sucking in cool ambient outdoor air, now it has
hot air literally thrown at it; coal needs air to burn.
Now the air is hot, not cold - where not as much coal needs to be burned as in 'hybrid coal-solar'.
To illustrate the power, the space shuttle was torn apart by hot air on reentry.

Most coal plants have heat recovery rotating 'thermal flywheels'
which take the heat of the exhaust, and uses it to make 'hot combustion air'.
Some of the burners would still get preheated air from the heat recovery turbine,
but some of the other burners would get their preheated combustion air from solar.

Sounds complicated? It is, but only a mechanical engineer would love the challenge.
The important point is this: converting a coal power plant to be
powered by solar energy is technically feasible. The big question is economics.

Other tricks of the trade exist to make hot combustion air for coal.

There is a method where a triple turbine compresses the hot air, making it even hotter.
One turbine vents high velocity air from the pipes. This drives a
second turbine that compress the hot air even hotter.
This then heats the air from the third turbine making that air much more hotter.
The net result is 1,000 degree hot combustion air
for combustion with coal. This means less coal is burned.

Wind and hydropower can also make hot combustion air.
Wind energy when windy, can compress and blow and heat air.
Hydropower also can heat, compress and blow hot air, when it rains.
Wind energy tends to be available at night. Hydropower is available in storms,
exactly when solar energy isn't. With close to 20 square miles of
land area consumed for a large solar farm, that is a large watershed.
That rainwater can generate hydropower, plus be sold to a
greenhouse farm to grow food, plants....and also make biomass.

Here is another way of converting a coal plant:
Biomass from greenhouses, or urban compost, yard-waste, and forest product residues
can be mixed with coal. This biomass has lower B.T.U.s and higher moisture than coal,
but the preheated solar air can make up for that loss.

Carbon Dioxide from the coal power plant can be bubbled through a lake of salt water, to grow algae.
Algae has large amounts of oil, made from sunlight. This oil can be
separated from the algae, and be mixed with the coal.
Again, integrating biomass with the fuel. Urban compost bio-gas
can be made from land fills, perhaps to power the pre-boiler.

Algae farming is a huge untapped agribusiness. Worldwide and underneath many deserts
like the Saharra, Asia, South Africa, Australia, and the American West is a
huge and ancient alkaline/salt water aquifer. Using solar and wind,
this impure water can be pumped to the surface of deserts to grow algae.
The oil production from algae farming could power all cars and aircraft. There is an added bonus:
The evaporation of rainwater from these salt water lakes, would add
moisture and fog to the desert...expanding the amount of
plant life in the deserts, and reducing desertification;
the coal can gradually be displaced with biomass burning.

Granted biomass burning does emit CO2 into the atmosphere, but plants do remove the
CO2 from the atmosphere. This makes for a natural biological loop, locking the CO2.

Eventually solar energy can be tapped to make hydrogen, either from electrolysis,
or from bacteria that convert water to hydrogen organically.
The hydrogen is then mixed with coal.

The combination of hydrogen, biomass, algae, greenhouse plants, algae-to-fuel,
urban compost, yard waste, and forest residues all add up.
When mixed with coal, it means less coal is burned.

Conversion of coal power plants to be powered by renewable energy doesn't happen overnight.
It will be a gradual conversion. Proposed is that the
first month 1% of the heat load be displaced, the second month 2%,
the third month 3%...and so on. This makes financing much easier.

The first 1% is revenue producing almost immediately.
This can help finance the next 2%. Unlike a nuclear power plant which will lock up
financing for nearly 5 years before it produces revenue,
solar conversion of hybrid solar-coal plants gives a sooner Return On Investment.

It's a fact that hot and compressed combustion air from solar
could be piped over mountains in heavily insulated pipes, just like we pipe water today.
The large number of underground pipes act as a 'thermal storage battery',
thus hot air is supplied 24 hours a day, 7 days a week, storing heat from
the day for night use, and storing heat from summer for winter use.

Another trick of the trade is to locate an industrial park
at each of the nation's coal power plants. Most coal plants have railroad tracks,
which could be used to rail in urban recyleables. Thrown away glass can be
converted to vacuum solar collectors. Aluminum cans converted to pipes.
Styrofoam can be mixed with coal flyash to make insulation. Urban compost can be processed
for mixing with coal. Plastic could be converted to diesel fuel for the trucks and trains.
Few know this, but at night there is a surplus of electrical power nationwide.
Nighttime, off-peak power would be used to process our garbage into solar collectors.

Power plants are in-fact huge consumers of electricity themselves.
It takes electricity to power the air pollution equipment, pumps, blowers, cooling towers,
and sometimes the dragline at the coal mine, using electricity knowing it takes electricity to make it.
This electrical parasitic load can be displaced with renewable energy.

Photovoltaics can make electricity on people's rooftops, while large wind turbines
can make power also. Run-of-the-River hydro turbines - which only operate when it rains,
can make electricity off urban storm water drains. To sum, each month 1% of the
parasitic electrical load would be displaced by renewable energy. The first month 1% of the electrical load
would displace, the next month 2%, the third month 3%, and so on.
This is a gradual conversion process.

I am a strong advocate of distributed power generation where buildings make their own energy,
but large bulk power generation will be needed for industrial factories.
Many factories simply can't be powered from the sunlight on roofs, so bulk power will have to be imported.
A combination of distributed solar and large scale solar will provide
the electrical transmission grid a reliable 24 hour supply.

Finding the land area for nearly 10,000 sq miles of solar collectors is a task, but not impossible.
For example, solar collector farms could be built on top of the
coal strip mine as part of the land reclamation process. Nationwide, the amount of
sunlight striking our parking lots can power the nation.
Oftentimes, the amount of sunlight falling on a hydro dam reservoir is more energy
than from the hydro dam. Finding the land for a solar farm to power coal plant
is a task, but not an impossible task. The goal is to build these
solar farms on 'brownland'...not pristine wilderness.

Can it be done? Yes! It would employ thousand of people.
It would use common skills like bricklaying, plumbing, electricians, and so on.
Plus it would clean up the atmosphere and provide a market
for all the garbage we throw away...
What's it cost?..
I suspect it will be no more expensive than the multi-billion dollar bank bail out.
Unlike banks, we get energy out.

Does coal have a future? I say yes, but not as a fuel.
Coal is better as a mineral, made into fertilizers, building materials, or useful chemicals.
It could be the oil of the future, but coal's future may not be fuel.
Plus, I would rather have people working and building Clean Solar instead of laboring in mines.
Thus I am proposing that the Clean Coal Act be modified
to include Solar Retrofit as part of the research.

This is all visionary. We need innovation to solve climate and related economic issues.
Conversion of coal power plants to renewable energy is feasible,
but the big question is the economics...not the physics.
It can be done - only to become a reality if 'we the people' can answer that question.. a certain starship captain hath said: 'let's make it so'!