Building Services Engineering 5th Edition Handbook

Hot- and cold-water supplies 197
UK produces a financial return on the capital invested in equipment of around 10% per annum
when electrical water heating is used. Solar energy is used to provide:
1. comfort heating through architectural design in a ‘passive’ system;
2. comfort heating using collectors, with air as the heat transfer fluid, in an ‘active’ system;
3. comfort heating and hot-water using collectors, with water as the heat transfer fluid, in an
active system.
Thermal storage of the collected energy is needed to balance the supply of solar heat with its
times of use, usually when the supply has greatly diminished. The following methods of thermal
storage are used:
1. thick concrete or brick construction for passive systems, combined with large areas of southoriented
glazing;
2. rock heat storage, heated by air recirculation;
3. water tanks;
4. phase change salts.
Flat-plate solar collectors are the most popular as they can heat water to 30–40 ◦ C without the
danger of boiling. They can be incorporated into the architectural design of a sloping roof and are
reasonably cheap. They are used to preheat the water supplied to the hot-water service storage
cylinder, as shown in Fig. 6.12. Only the simplest arrangement is indicated, where the cold-water
Roof line
4 m 2 flat-plate
solar collector
supported on
rafters
Return
Flow
Temperature detector
Differential water
temperature controller
operates pump
Main
Overflow
Solar pump
Preheated water storage
tank for hot-water
service system
Cold
feed
Hot-water storage
cylinder
Vent
Hotwater
supply
to taps
Primary flow
and return
from boiler
6.12 Solar collector to preheat a hot-water service system.