Part B3: System types

B3.1System types Classification TypeNotesTypeNotes StandaloneLoad 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) PassiveNo source of conventional power. No moving parts –Thermosiphon water systems –Passive space heating ActiveNeeds pumps or fans DomesticGenerally need storage as load does not match availability Tend to want energy at night and early morning IndustrialMust be high reliability. Load is very predictable LiquidAir No storeE.g. crop dryers, solar cookers, some passive solar Daily storeMost space heating and hot water systems Seasonal storeDomestic space heating e.g. heat pumps in Sweden

B3.1System types Classification Direct Indirect

B3.2System types Thermosyphon system

g = gravity (ms -2 )  h = density of the heated water (kg/m 3 )  c = density cold water (kg/m 3 ) h = height (m) m = Mass flow rate (kg/s) k = constant

B3.2System types Thermosyphon 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

B3.3System types Pumped 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 T c 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

B3.4System types Pumped indirect system THTH T tank

B3.4System types Pumped 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/m 2 /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

B3.5System types Complete closed system

Bypass is a 3 way valve –Make T xo – T Li if possible (modulating) –Total bypass if T s < T xi Auxiliary heater modulates to keep T Li 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

B3.5System types Complete closed system Aux heater in series (topping up type) Aux heater in parallel (all or nothing type)

B3.5System types Complete closed system

B3.6System types system without storage

B3.7System types Multiple collectors

B3.8System types Design 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 F R Flow rate of less than 1 m s -1 avoids problems of erosion and noise

B3.8System types Design 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

B3.8System types Design points Fluids –Water –antifreeze (if allowed by water regs) –Corrosion inhibitors may need regular replacement as they are not stable at high temps Temperature sensors –Ensure good contact –Thermostats will have hysterisis – plan toping up cycle accordingly