"Power Plant" Takes on a New Meaning
Sep 26 - Machine Design
Waste heat from a 100-MW electric plant could, in theory, warm 100 acres of greenhouses.
About 15% of tomatoes consumed in the U.S. come from greenhouses, and that
number is rising. One reason: Yields for greenhouse-grown fresh produce are
about 10 times that of open fields. Not to mention, imported tomatoes are
expensive, especially in off-season. Transatlantic shipping costs in most cases
exceed the cost of the tomatoes. Then there are the growing concerns of food
safety and bioterrorism associated with imported crops.
Of course greenhouses require heat and (ideally) a carbon- dioxide (CO2)-rich
atmosphere to accelerate growing, jobs mostly handled today by dedicated and
costly natural-gas-fired boilers and, in some cases, cryogenic CO2 systems.
But there is another source of CO2 and heat that is now literally going up in
smoke: electric power plants. Flue gas from fossil-fuel- fired power plants is
rich in CO2. The burning of 1 m^suP 3^ of natural gas makes about 2-kg CO2 or
10% by volume, for example. A controlled amount of flue gas can be vented to a
greenhouse, while waste heat from electricity generation provides heating.
Green- houses would colocate with power plants to reduce thermal and piping
losses as well as tap electric power for artificial lighting, yet another
requirement for growing tomatoes and other crops during winter days and feeble
sunlight.
That is the concept being forwarded by The Greenhouse Corp. of America (GCA),
Newton, Mass. GCA plans to team with power-plant designer/builder Calpine Corp.,
San Jose, and improve upon technology developed in the Netherlands.
The idea of using waste heat to warm greenhouses isn't new. Since 1987 the
13-acre Cologne 6 greenhouse in Germany has been heated entirely with
cooling-tower water from the nearby Niederau em coal- fired 2,700-MW power
plant. A system of water-to-air heat exchangers with computer-controlled forced
ventilation maintains an indoor temperature up to 72F when outside temperature
dips to 7F. Water- inlet temperature ranges from 86F in winter to 104F in
summer.
GCA greenhouse heating and CO2-enrichment system hooked to a combined-cycle,
gas-fired power plant
Combined-cycle, gas-turbine plants use exhaust from a gas turbine to make
steam which, in turn, powers a steam turbine. Both turbines drive generator
sets. Artificial lighting would consume about 0.3 MW/ acre for 10 hr/day during
winter months or about 3,600 MW-hr for a 10-acre greenhouse. Total annual cost
is $ 180,000, assuming power comes directly from a power plant and at a unit
cost of $0.05/kW- hr. However, the ability to harvest crops in winter may bring
an additional $ 1 million in revenue.
This relatively tepid water requires huge heat exchangers to make the system
work. For comparison, a 10-acre greenhouse run by Garden State Growers in Home
City, Pa., on average gets about 30% of its heat from 110F cooling-tower water
of a nearby 2,000-MW power plant. The plant provides the hot water at low cost
in exchange for a tax credit. Dedicated gas-fired boilers supply the rest of the
energy. Still, the savings can be substantial. Last December, for instance, the
bill for supplemental gas totaled $56,000 because the pipeline carrying the hot
water was out of commission. Gas cost was only $8,000 the year before when the
system worked.
However, more practical systems need water inlet temperatures of about 176 to
194F. Unfortunately cooling-tower water from modern combined-cycle power plants
such as one in Westbrook, Maine, is only 80 to 90F. And many combined-cycle
plants are air cooled so there is no cooling water whatsoever. Yet another
approach uses small (40 to 60 MW), dedicated cogeneration plants that
simultaneously output electricity and hot water. "But greenhouses don't
need both resources all of the time so there is wasted capacity," explains
GCA founder Moshe Alamaro. Larger cogeneration plants such as the ROCA3 plant in
the Netherlands provide 220 MW of electricity and 200 MW of heat plus CO2 to
greenhouses up to 15 miles away. But moving the resources over long distances
tends to be costly.
STEAM ON DEMAND
The GCA design avoids these shortcomings. Here's how it works: Steam from the
low-pressure inlet of a power-plant steam turbine pipes to a quencher. The
quencher combines the steam with water to release latent heat. The heated water
circulates through forced-air heat exchangers, then a small portion loops back
to the power plant's heat-recovery and steam-generation sections after passing
through a reverse-osmosis unit. Make-up water also passes through the ROU.
Unlike dedicated cogeneration systems, growers pay for steam only when it is
needed. Greenhouses share land with the power plants, which lowers
infrastructure and operating costs.
Accumulation of harmful gases in greenhouses when using contaminated CO2 for
enrichment
Greenhouse heating needs
A cumulative frequency curve shows typical annual heating needs of a
greenhouse used to grow tomatoes.
Heat gain to a greenhouse equals minus losses, of course - the enthalpy
difference between steam at the turbine tap and quencher, a relatively large
number because the steam changes phase. In theory, assuming saturated vapor and
liquid, the ratio of greenhouse heating- toturbine-power-loss is about 20:1. In
practice, steam vapor at the turbine tap is superheated so its enthalpy is
higher. Moreover, steam has less enthalpy at the condenser than idealized
systems so actual heating-to-loss ratios are about 6 or 7:1. Heating a typical
20-acre greenhouse in winter - assuming an outside air temperature of 5F and a
200-W/m^sup 2^ heat loss from the greenhouse - requires about 16 MW of thermal
energy or 55.3 million Btu/hr. Dividing thermal energy by 7 gives the reduction
in power output from the power plant, or about 2.3 MW.
GOOD GAS, BAD GAS
The GCA system works best with modern, gas-fired power plants because
unwanted contaminants in flue gas from plants burning other types of fuels can
be difficult and costly to filter out. Gas-fired plants would as well require
scrubbers on smoke stacks to remove phytoxic gases. Phytoxics can severely
damage plants, even in small amounts. Ethylene, and to a lesser degree sulphur
dioxide and NOx (nitric oxide plus nitrogen dioxide), are phytoxic.
Fuels containing high concentrations of sulphur such as coal, wood, and heavy
oil generate significant amounts of sulphur dioxide when burned, making them
unsuitable for CO2 enrichment in greenhouses. Fossil fuels that are virtually
free of sulphur include pure propane, butane, LPG, natural gas, premium
kerosene, and low- sulphur oil. However, sulphur-free fuels may also generate
NOx, though burners that better control fuel-to-air mixtures and combustion
temperatures significantly lower these emissions. Here, regulation has driven
much of the improvement in burner technology.
Incomplete combustion in low oxygen conditions is another source of
harmful-to-plants flue gases, including carbon monoxide, nitric oxide, and
ethylene. Ethylene is especially problematic because it acts as a hormone that
stimulates plants to ripen, mature, age, and rot. It is generally agreed that
ethylene levels of about 10 to 20 parts per billion (ppb) or higher can damage
plants. For reference, ambient ethylene concentrations are about 1 to 5 ppb but
can be much higher in industrial areas and population centers with large numbers
of motor vehicles.
Yields for greenhouse-grown produce are about 10 that of open fields. 5
Obviously CO2 is the only useful component of flue gas for use in
greenhouses. CO2 concentrations of about 1,000 ppm dramatically boost plant
growth rates. Ambient CO2 levels are typically 350 to 400 ppm, for reference.
Plants absorb CO2 in photosynthesis so the gas must be continuously replenished
to maintain proper levels inside greenhouses, either through enrichment or
ventilation. Harmful gases, on the other hand, are not absorbed by plants and
tend to accumulate. The accumulation of these gases depends on several factors
including their percent concentration relative to CO2, CO2 injection rate, and
losses from greenhouse ventilation and leakage.
For example, say CO2 enrichment influx = 0.7 m^sup 3^/m^sup 3^ and the
concentration of harmful gases = 1 ppm of CO2 influx. Assuming no greenhouse
leakage, concentration of harmful gases would reach 7 ppb after 10 hr. Factoring
in a ventilation loss of 0.5 m^sup 3^/m^sup 3^/hr balances harmful-gas influx
and efflux at 1.4 ppb. Under these conditions, harmful gas concentration reaches
0.8 ppb after 2 hr and 1.1 ppb an hour later then rises slowly to a steady-state
1.4 ppb. Ethylene concentration after 6 hr is of particular interest because
this duration corresponds to CO2 enrichment on a winter day.
SEED CAPITAL
If all goes as planned, GCA will first develop a 20-acre greenhouse with
power plant heating and CO2 enrichment. Estimated cost: $7 to $10 million. The
design and size would be standardized to ease expansion. Eventually greenhouses
would be franchised to local growers with equipment shared among regional
locations.
The time may be right. "Recent studies suggest that the U.S. will nee\d
an additional 6,000 acres of greenhouses to satisfy domestic tomato demand in
the coming decade," says Alamaro.
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Lawrence Kren
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Copyright Penton Media, Inc. Sep 16, 2004