Solar Thermal Information

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• Collector flow rate
• Storage tank volume
• Effectiveness of collector/storage heat exchanger
• Specific heat of collector circuit fluid
• Space heating load
• Domestic hot water load
Once a specific system is defined by these inputs, f-chart
calculates several outputs on a monthly and annual basis.
These include:
• Total solar radiation incident on the collector area
• Space heating load of the building
• Domestic water heating load of the building
• Auxiliary energy needed for space heating and DHW
• Percentage of the monthly (space heating + DHW) load
supplied by solar
An output screen listing these outputs for a specific
combisystem is shown in figure 6-1. The lowest line is the
annual total of the monthly quantities listed above it, except
for the solar fraction (f), in which case the lower line gives
the annual solar fraction. For the results given in figure 6-1,
solar energy supplied 0.235 (e.g., 23.5%) of the annual total
space heating plus domestic water heating load.
CASE STUDIES:
The performance of some representative solar combi-
systems will now be discussed. These combisystems
have been designed for two different houses: one with a
design heating load of 35,000 Btu/hr and the other with a
design heating load of 100,000 Btu/hr.
Each house is assumed to be located in Syracuse, New
York (a cold and relatively cloudy winter climate), as well
as in Colorado Springs, Colorado (a cold but relatively
sunny winter climate).
Each house has been analyzed with two different
combisystems: One built around four 4-foot by 8-foot
flatplate collectors with 256 gallons of water storage, and
the other built around eight of the same collectors and
512 gallons of storage.
Other specific data for each system is as follows:
For the smaller house:
• Design heating load = 35,000 Btu/hr, with outdoor
temperature = 0ºF and indoor temperature is 70ºF
• Domestic water heating load = 60 gallons per day
heated from the local cold water temperature to 125ºF.
For the larger house:
• Design heating load = 100,000 Btu/hr, with outdoor
temperature = 0ºF and indoor temperature is 70ºF
• Domestic water heating load = 100 gallons per day
heated from the local cold water temperature to 125ºF.
• Each system uses drainback freeze protection with no
collector-to-storage heat exchanger
• Collectors are 4-foot by 8-foot, with a gross area of 32
square feet each
• Collector efficiency intercept is 0.76, and the efficiency
slope is 0.865 (Btu/hr/ft
2
/ºF)
• Collector arrays are sloped at local latitude +15º
• Collector arrays face directly south
• Storage volume is 2 gallons per square foot of collector
area
The performance estimates that follow
were all determined using f-chart
software.
Figure 6-2 shows the monthly solar
fractions for both combisystems in the
smaller house located in Syracuse, NY.
Notice that several bars representing the
solar fraction in mid-year reach 100%,
even for the smaller combisystem. This
happens because the collector array
can generate more heat than the load
requires at this time of year. During the
colder months, the larger system has
approximately double the solar fraction
of the small system. However, on an
annual basis, the four-collector system
meets 23.5% of the total load of the
smaller house, whereas the 8-collector
figure 6-1