Systems Solar water heating

1 systems

1.1 heat transfer

1.1.1 direct
1.1.2 indirect


1.2 propulsion

1.2.1 passive
1.2.2 active


1.3 passive direct systems
1.4 active indirect systems
1.5 do-it-yourself (diy)
1.6 comparison

systems

sample designs include simple glass-topped insulated box flat solar absorber made of sheet metal, attached copper heat exchanger pipes , dark-colored, or set of metal tubes surrounded evacuated (near vacuum) glass cylinder. in industrial cases parabolic mirror can concentrate sunlight on tube. heat stored in hot water storage tank. volume of tank needs larger solar heating systems compensate bad weather , because optimum final temperature solar collector lower typical immersion or combustion heater. heat transfer fluid (htf) absorber may water, more commonly (at least in active systems) separate loop of fluid containing anti-freeze , corrosion inhibitor delivers heat tank through heat exchanger (commonly coil of copper heat exchanger tubing within tank). copper important component in solar thermal heating , cooling systems because of high heat conductivity, atmospheric , water corrosion resistance, sealing , joining soldering , mechanical strength. copper used both in receivers , primary circuits (pipes , heat exchangers water tanks).

another lower-maintenance concept drain-back . no anti-freeze required; instead, piping sloped cause water drain tank. tank not pressurized , operates @ atmospheric pressure. pump shuts off, flow reverses , pipes empty before freezing can occur.

how solar hot water system works

residential solar thermal installations fall 2 groups: passive (sometimes called compact ) , active (sometimes called pumped ) systems. both typically include auxiliary energy source (electric heating element or connection gas or fuel oil central heating system) activated when water in tank falls below minimum temperature setting, ensuring hot water available. combination of solar water heating , back-up heat wood stove chimney can enable hot water system work year round in cooler climates, without supplemental heat requirement of solar water heating system being met fossil fuels or electricity.

when solar water heating , hot-water central heating system used together, solar heat either concentrated in pre-heating tank feeds tank heated central heating, or solar heat exchanger replace lower heating element , upper element remain provide supplemental heat. however, primary need central heating @ night , in winter when solar gain lower. therefore, solar water heating washing , bathing better application central heating because supply , demand better matched. in many climates, solar hot water system can provide 85% of domestic hot water energy. can include domestic non-electric concentrating solar thermal systems. in many northern european countries, combined hot water , space heating systems (solar combisystems) used provide 15 25% of home heating energy. when combined storage, large scale solar heating can provide 50-97% of annual heat consumption district heating.

heat transfer
direct

direct systems: (a) passive chs system tank above collector. (b) active system pump , controller driven photovoltaic panel.

direct or open loop systems circulate potable water through collectors. relatively cheap. drawbacks include:

they offer little or no overheat protection unless have heat export pump.
they offer little or no freeze protection, unless collectors freeze-tolerant.
collectors accumulate scale in hard water areas, unless ion-exchange softener used.

the advent of freeze-tolerant designs expanded market swh colder climates. in freezing conditions, earlier models damaged when water turned ice, rupturing 1 or more components.

indirect

indirect or closed loop systems use heat exchanger transfer heat heat-transfer fluid (htf) fluid potable water. common htf antifreeze/water mix typically uses non-toxic propylene glycol. after heating in panels, htf travels heat exchanger, heat transferred potable water. indirect systems offer freeze protection , typically overheat protection.

propulsion
passive

passive systems rely on heat-driven convection or heat pipes circulate working fluid. passive systems cost less , require low or no maintenance, less efficient. overheating , freezing major concerns.

active

active systems use 1 or more pumps circulate water and/or heating fluid. permits wider range of system configurations.

pumped systems more expensive purchase , operate. however, operate @ higher efficiency can more controlled.

active systems have controllers features such interaction backup electric or gas-driven water heater, calculation , logging of energy saved, safety functions, remote access , informative displays.

passive direct systems

an integrated collector storage (ics) system

an integrated collector storage (ics or batch heater) system uses tank acts both storage , collector. batch heaters thin rectilinear tanks glass side facing sun @ noon. simple , less costly plate , tube collectors, may require bracing if installed on roof (to support 400–700 lb (180–320 kg) lbs of water), suffer significant heat loss @ night since side facing sun largely uninsulated , suitable in moderate climates.

a convection heat storage unit (chs) system similar ics system, except storage tank , collector physically separated , transfer between 2 driven convection. chs systems typically use standard flat-plate type or evacuated tube collectors. storage tank must located above collectors convection work properly. main benefit of chs systems on ics systems heat loss largely avoided since storage tank can insulated. since panels located below storage tank, heat loss not cause convection, cold water stays @ lowest part of system.

active indirect systems

pressurized antifreeze systems use mix of antifreeze (almost non-toxic propylene glycol) , water mix htf in order prevent freeze damage.

though effective @ preventing freeze damage, antifreeze systems have drawbacks:

if htf gets hot glycol degrades acid , provides no freeze protection , begins dissolve solar loop s components.
systems without drainback tanks must circulate htf – regardless of temperature of storage tank – prevent htf degrading. excessive temperatures in tank cause increased scale , sediment build-up, possible severe burns if tempering valve not installed, , if used storage, possible thermostat failure.
the glycol/water htf must replaced every 3–8 years, depending on temperatures has experienced.
some jurisdictions require more-expensive, double-walled heat exchangers though propylene glycol non-toxic.
even though htf contains glycol prevent freezing, circulates hot water storage tank collectors @ low temperatures (e.g. below 40 °f (4 °c)), causing substantial heat loss.

a drainback system active indirect system htf (usually pure water) circulates through collector, driven pump. collector piping not pressurized , includes open drainback reservoir contained in conditioned or semi-conditioned space. htf remains in drainback reseervoir unless pump operating , returns there (emptying collector) when pump switched off. collector system, including piping, must drain via gravity drainback tank. drainback systems not subject freezing or overheating. pump operates when appropriate heat collection, not protect htf, increasing efficiency , reducing pumping costs.

do-it-yourself (diy)

plans solar water heating systems available on internet. diy swh systems cheaper commercial ones, , used both in developed , developing world.

comparison