Part B3: System types

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

B3.1 System typesClassification

Direct

Indirect

B3.2 System typesThermosyphon system

B3.2 System typesThermosyphon system

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

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

TH

Ttank

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

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)

B3.5 System typesComplete closed system

B3.6 System typessystem without storage

B3.7 System typesMultiple collectors

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

B3.8 System typesDesign 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