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These flow rates were chosen based on the collection area of each solar collector and the incoming solar
radiation at design conditions. The flow rates allow the collectors to achieve an 18 degree Fahrenheit temperature
rise across the solar loop.
‘The sun is the source, the system is the battery.’
Solar energy is only available for a limited time during each day. It is because of this that the energy must be
captured and stored for later use. The amount of energy stored is related to the system operating temperature
(i.e. radiant floor heat at 100F or fan coil units at 180F), the storage volume, and to the maximum temperature in
the storage vessel. Maximum energy storage [BTU’s] is calculated using equation 1, below.
Maximum Stored Energy = 8.33 x Storage volume [Gal] x (Max. tank temp. – System operating temp.)
Where, Storage volume is in gallons and all temperatures are in Fahrenheit. Optimization of this equation has
determined the design number of gallons per collector. The table below shows the number of gallons of storage
recommended per collector.
Collector Type
Gallons of Storage per Collector
OV 5-16 Evacuated tube collector
66 Gallons
OVF-21 Flat plate collector
27 Gallons
OVF-32 Flat plate collector
41 Gallons
OVF-40 Flat plate collector
53 Gallons
These storage values are calculated for a national average. Since daily solar radiation varies significantly from the
South to the North, these storage values are not required, only recommended. Less storage can be used in the
northern latitudes and more storage can be used in the southern latitudes. The storage values should only vary plus
or minus 5% from the recommended values listed above.
Another way to think of solar storage is as the fuel tank of the system. It is used to store energy that can be used
over the entire day. In this way the daily load is matched to the daily collector output. Oventrop solar hot water
systems are designed to optimize the transfer of energy from the collectors to the solar storage vessel.
Small systems
Internal coil tanks are used for Oventrop smaller systems. This allows for the ease of installation for system
packages. Oventrop has pre-packaged solar domestic hot water systems for daily loads of 80 and 115 gallons.
Depending on location these packaged systems will provide between 30 and 70 percent of the annual domestic hot
water load.
Using a similar configuration as the pre-packaged systems, larger domestic hot water loads can be accommodated.
Variations of small systems are sized by tank size and can be arranged for up to 160 gallons and 230 gallons of daily
usage. These small systems are comfortably in the design range of the Regusol EL 130 solar pumping station. This
pumping station is designed for a maximum flow rate of four (4) gallons per minute. Because the Regusol EL 130 is
constructed in ¾”, the maximum flow rate is based on the maximum trouble free flow through ¾” pipe.
Medium systems
Medium systems are above the flow range established for the Regusol EL 130 and have increasingly larger storage
requirements. It is for these reasons that the Regusol-X pumping station with heat exchanger was developed.
External heat exchangers provide flexibility of design and reduce cost when considering larger storage volume and
collector arrays. The Regusol-X is designed to be used with any storage vessel that is built for hot water storage; it
does not require internal heat exchangers.