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B3.1 System typesClassification
Type Notes Type Notes
Standalone • Load may not be met all the time
• Solar energy is sole input
• Cheap and simple
Solar supplemented
• Auxiliary source tops up solar as required
• Solar system acts as a fuel economiser (most systems are like this)
Passive • No source of conventional power. No moving parts
– Thermosiphon water systems
– Passive space heating
Active • Needs pumps or fans
Domestic • Generally need storage as load does not match availability
• Tend to want energy at night and early morning
Industrial •Must be high reliability.
•Load is very predictable
Liquid Air
No store • E.g. crop dryers, solar cookers, some passive solar
Daily store • Most space heating and hot water systems
Seasonal store • Domestic space heating e.g. heat pumps in Sweden
B3.2 System typesThermosyphon system
g = gravity (ms-2)h = density of the heated
water (kg/m3)c = density cold water
(kg/m3)h = height (m)m = Mass flow rate (kg/s)k = constant
2 turbulent
laminar h cg dh m k
mk
m
B3.2 System typesThermosyphon system
• Pumping only occurs when panel is hot
• Tank must be above collector
• No freeze protection– good for hot climate– Draindown necessary in cold climate with a
direct system or antifreeze in indirect
• Pressure change can cause problems so pipe should be large bore
• Air locks will stop the system so the pipe must rise constantly
• The system must include a check valve to stop reverse Thermosyphon
• T typically 10°C to get a flow
B3.3 System typesPumped direct system
• Collector performance is independent of store temperature
• Tank can be below panel
• Used when when T across load is high e.g. industrial process heat
• Because Tc fixed over a day, easier to predict performance
• Tend to be more efficient over the year than closed system – if they can be used at all
B3.4 System typesPumped indirect system
• Needs antifreeze in the UK
• Control system needed to turn on pump when Panel outlet temperature exceeds tank temperature + T
• Flow around 50 kg/m2/hr
• 5 – 10 passes throughout on a sunny day
• T typically 2 – 5°C per pass
• The system must include a check valve to stop reverse thermosyphon
• Should have a vent or expansion tank
B3.5 System typesComplete closed system
• Bypass is a 3 way valve– Make Txo – TLi if possible (modulating)
– Total bypass if Ts < Txi
• Auxiliary heater modulates to keep TLi constant– Not always possible due to expense or retrofit– If modulating heater is not available than use a
parallel auxiliary heater which will• Have All or nothing operation• Be less efficient• Try to avoid rapid cycling of the auxiliary heater as
it reduces efficiency
• Needs relief valves
B3.5 System typesComplete closed system
Aux heater in series
(topping up type)
Aux heater in parallel
(all or nothing type)
B3.8 System typesDesign points
• Corrosion– Bi-metalic at interface
• Use inhibitors or gun metal couplings
– Pitting: Metal around carried by electrolyte.• Use inhibitors and biocides
• Avoid leaks/air locks
• Flow rate of more than 0.02 kg s-1 m-2 for water gives a good FR
• Flow rate of less than 1 m s-1 avoids problems of erosion and noise
B3.8 System typesDesign points
• Vent pipes needed on unsealed systems with a free path to allow for expansion
• Pressure relief valves (and gauges) on sealed systems
• Regulations– Building codes– Fire regs– Water company regs/guidelines