Horticulture Principles and Practices

FIGURE 6 Amounts of waste
energy utilized in various size
greenhouses. (Source: A.J. Both and
D.R. Mears)
technical feasibility and identify key design relationships, (Ekholt et al, 1983 and Both
et al, 2007). Using performance data from a small cogeneration unit designed to operate
on landfill gas the first study was done to compare the relative effectiveness of floors with
and without the flooded storage. It was found that the larger storage capacity provided
more management options for both the sizing of the cogeneration unit relative to the electrical
load of the greenhouse and the periods of operation of the unit to realize optimum
economics. The later study investigated alternative warm water storage systems and
capacities for more flexibility in matching specific greenhouse operations.
Additional concepts modeled include the use of an industrial scale fuel cell
designed to operate on natural gas. With sufficient storage it was found that matching
such a unit to an appropriately sized greenhouse will enable a half or more of the total
waste heat put out by the unit over a year to be utilized in the greenhouse. In addition
there is the opportunity to capture the carbon dioxide that is produced by the unit as it
first converts methane to pure hydrogen for the fuel cell exhausting the carbon component
as carbon dioxide. The key barrier to economic application of such a unit remains
high initial cost. Figure 6 illustrates the amounts of the waste energy generated by a 200
kW fuel cell that can be utilized in various size greenhouses. Having significant capacity
to store heat enables the greenhouse to use the full daily output when heat is only
needed at night. The weather database used for the calculations in the figure is a composite
constructed from ten years of records at the Philadelphia International Airport.
Another concept investigated is the use of a heat pump for environmental control in a
greenhouse. With the continuing rise in the cost of gas and oil relative to electricity noted
previously the use of a heat pump, which may generate three to six times the output relative
to electricity input, would become economically competitive as the operating cost savings
amortize the initial investment. An efficient system can incorporate a water-to-water heat
pump using groundwater as a source for heating and/or a sink for cooling. As the equipment
is relatively expensive there is significant economic advantage in using energy thermal storage
so a smaller unit can run day and night with the heat generated during the day available
for night heating. Such a system should be designed to meet only base load heat requirements
with a lower cost backup system on gas or oil utilized for peak requirements.
As heat pumps provide cooling capacity as well as heating another mode of operation
can be considered in which the heat pump provides early stage cooling to the greenhouse.
With storage for cool and warm water the heat pump can cool the greenhouse during the day
in fall, winter and spring storing the heat for use at night. A significant advantage of such an
approach is that the greenhouse may be kept closed longer during periods when only early
stage ventilation is needed and ventilation can be reduced under hotter conditions thereby
extending the time that carbon dioxide enrichment can be economically employed. Figure
7 illustrates the contributions to the heat requirements that are predicted for various months
for a proposed system cooling the greenhouse when needed but drawing on a well at other
times to maximize its utilization. The large, energy efficient greenhouse was projected to
12.3 Internal Environmental Control 415