Viessmann VITOSOL 200-FM SV2F Design Manual page 62

Design and Operation
Sizing the Solar Thermal System
Domestic hot water pre-heat systems
Solar thermal systems have the ability to considerably
reduce the energy requirements of the backup heat
generator, assuming that the building has a constant daily
DHW load throughout the year. As a result solar thermal
systems are well suited to almost all residential and high
occupancy apartment buildings.
Using the formula below, it is possible to quickly identify
the number of Vitosol 200-FM collectors which are
required to provide 100% of the monthly domestic hot
water load for a residential household.
For this sizing calculation, the user must select a
location from the list (see pages 64 and 65). Since solar
availability is greater during the summer months than it
is during the winter, it is recommended that calculations
be completed for each month to identify the appropriate
number of collectors to meet the monthly DHW demand.
Note: To speed up the calculation process, calculate for
the months of June & July. Since these months
receive the greatest amount of solar radiation,
as a result they will require the least number of
solar collectors to satisfy the DHW load.
Nc = ((Ndays x Npersons x Vperson x p x Cp x ∆T) ÷ Se)
÷ (Gt x 1,000,000 x Ndays) ÷ (Ca)
Where:
Nc = Number of collectors required to meet 100% of
the domestic hot water load
Ndays = Number of days per month (see table below)
Npersons = Number of people in the home or building
Vperson = Volume of hot water consumed per person
each day (50-75 Liters per person per day)
p = Density of water (1 kg/L)
Cp = Specific heat of water (4190 J/kgºC)
∆T = Temperature difference between hot water to
be delivered from the storage tank and the cold
water coming in from the well or city mains
(50 ºC rise)
Se = System efficiency in decimal format
(assume 70% collector efficiency x 60% balance
of system efficiency = 42% or 0.42)
Gt = Solar radiation (MJ/m²/day)
*Note: 1 kwh = 3.6 MJ or 3,600,000 Joules
Ca = Collector absorber area (Vitosol 200-FM = 2.32 m²)
Number of days in each month
January = 31
February = 28
March = 31
April = 30
May = 31
June = 30
62
(continued)
July = 31
August = 31
September = 30
October = 31
November = 30
December = 31
Calculation example for Vitosol 200-FM
Number of days per month: ...................30 days in June
Number of people in the home: ....................................4
Volume of hot water consumed
per person per day: ....................................50 L/per/day
Density of water: ................................................1 kg/L
Specific heat of water: ................................4190 J/kgºC
Temperature difference between hot water tank
and cold water supply: ..........................................50°C
System efficiency: .................................................0.42
Solar radiation (Boston): .........................22.1 MJ/m²/day
Collector absorber area (Vitosol 200-FM): ............ 2.32 m²
Nc = ((30 days x 4 people x 50 L/per/day x 1 kg/L x
4190 J/kgºC x (50ºC)) ÷ 0.42) ÷ (22.1 MJ/m²/day
x 1,000,000 x 30 days) ÷ (2.32m²)
= 1.95 Vitosol 200-FM collectors
Note: To prevent overheating it is recommended to
round the quantity of collectors down to the
next whole number. For this example, 2x Vitosol
200-FM collectors would be the recommended
number of collectors to satisfy the DHW load for
the month of June.
Solar System Design Guide