Domestic solar water heating

Domestic solar water heating

The sun is the greatest energy generator of our solar system. Energy is provided freely to the whole of the planet although not evenly distributed. A considerable level of unpredictability especially in earth’s higher and lower latitudes is an important issue. Never the less, solar energy can decrease considerably the amount of energy consumed for domestic water usage as well as space heating through solar collectors connected to water tanks placed usually on a dwelling’s roof area. Solar water heating

Specifically, solar water heating for domestic use means simply converting solar energy into thermal to increase water temperature to appropriate levels. This is a very mature technology utilized for the last 80 years.

Solar water heating

Panel types

Flat plate panels:
This type is the most widely used. It is consisted of a glazed insulated panel including an absorber surface. This converts solar to thermal energy which is then conducted into the pipes.

Evacuated tubes:
These collectors consist of a number of parallel connected glass tubes. Most popular types are: Panel

Glass-glass created out of tubes fused to each other on one end. A selective coating on the inner tube absorbs most of the solar radiation available. This is converted to heat locked inside the tubes as no conduction or convection is possible due to the air withdrawal between the pipes.

Glass-metal tubes are single evacuated tubes with an aluminium curved plate attached to a copper pipe in the inner part which are very efficient but may present some vacuum loss issues as their seal is glass to metal.

Glass-glass tubes may not be as efficient but are more reliable and have a lower cost.

Evacuated (or vacuum) tubes panel

Panel efficiency:
Evacuated tubes seem to maintain a higher yearly average efficiency than flat plate panels especially when Tm-Ta (temperature difference between water in the collector and the ambient) becomes higher than 25 degrees C. They also provide us with significantly more energy under overcast skies.
Panel efficiency [d]

Solar system types

Thermo siphon systems Thermo-siphon systems based on gravity: These systems are more common in Greece, Cyprus and Israel. They are cheap and are considered adequate for warm climates where freeze protection is not usually required though not as efficient.

Direct system
In such systems potable water is pumped through the panel for direct heating. The system is switched off under overheating or freezing conditions. Plastic pipes however may tolerate some frost. Water softening may be required as to avoid corrosion in the systems circulation. Finally legionella might be an issue and should be controlled.

Indirect systems
In this case potable water is heated by a heat exchanger. A fluid is pumped, consuming a low amount of electricity (about £7 per year). Heat exchange systems are further divided in those that drain back and those that do not.
In those that do, the tank is not pressurized and when the pump shuts off, the flow reverses with the pipes being emptied before freezing.
Those that do not, are proven reliable and are considered a mature technology. They are filled up with a mixture of glycol propylene functioning as an anti freezer. Such systems are able to provide hot water in cold as well as hot conditions and are most popular in northern Europe.
Active closed loop solar water heater [a]

Sizing

Hot water demand
Average values of domestic hot water usage per day are illustrated in the following table:

Energy consumption for domestic hot water of a three-person family

Appliance/use

DHW load consumption
KW h/day

Bath/shower	1.1
Wash hand basin	1.4
Dish washing	2.3
Clothes washing 50%	2.0
Clothes washing 50%	0

Output temperature (°C); Tin the water input temperature (10 °C)

[c]

These would add up to 9kWh/day for a 4 person home or an average of 3300kWh/year

Climatic data

Local climatic conditions are important in calculating our solar gains:
Ambient temperature influences significantly heat losses.
The collectors must be oriented south (northern hemisphere) and be set at the optimum inclination angle. Solar irradiation on such an inclined surface is roughly 950 kWh/m2 per year in North UK (Scotland) to about 1950 kWh/m2 in Palermo.

Optimum inclination solar radiatin values per m2 surface area optimally inclined are based on following map

1000 kwh/year Scotland ->2.5 kwh/day
1950 kwh/year Southern Italy->5 kwh/day

Photovoltaic solar electricity potential in European countries

Photovoltaic solar electricity potential in European countries [e]

Panel size

3-4 m2 should be adequate for a 4 person home varying in relation to the use of different types of panels (flat solar collectors require usually bigger roof area than evacuated tubes).
Tank sizes
When choosing cylinders connected to solar panels its important to choose a 20-30% bigger model. The quantity of water warmed in the cylinder shall be less than that of a conventionally heated cylinder. Over sizing the cylinder again would result to lower than required average cylinder temperatures [h].
Tank stratification in order to optimise efficiency:
High thermal stratification can increase performance due to the lower return to the panel meaning efficiency increase.
Mean load for a home of 4 occupants would mean roughly 200 l/day.

System efficiencies

Assuming a linear relation between average solar radiation values and panel efficiencies, estimations included for northern and southern Europe were added in the following table based on literature: Collectors-flatplate-evactube [i]

Solar thermal systems daily energy generation. Evacuated tube systems considered have 20 tubes
Panel Type	Flat plate	Flat plate	Flat plate	Evac tube	Evac tube
Configuration	Direct active	Thermosiphon	Indirect active	Indirect active	Direct active
Total area (m2)	2.49	1.98	1.87	2.85	2.97
Absorber area (m2)	2.21	1.98	1.72	2.85	2.96
Max efficiency	0.68	0.74	0.61	0.57	0.46
Energy generation (kWh/day):
Insolation 2.5 kWh /day (north Europe)	4.0	2.9	2.5	3.7	3.1
Insolation 3.2 kWh /day (mainland Europe)	5.3	3.9	3.3	4.8	4.0
Insolation 5 kWh /day (south Europe)	8.5	6.3	5.4	7.5	6.3
Insolation 6.5 kWh /day (tropic regions)	11.2	8.8	7.1	9.9	8.4

[b]

Energy conversion efficiencies between panel types are generally small. Durability and long lasting-low maintenance are the key issues under consideration (EN12975 1&2).

Utilisation factor

‘UF’ is the percentage of available solar energy actually consumed regarding DSWHS efficiency and daily- seasonal hot water demand patterns. According to numerous consumer profiles and solar radiation combinations assessed ‘UF’ basically varies from 25 to 40% [f].

Costs- payback time

In the UK systems vary from £2,000 to £6,000 [g],[f] depending on:

size of dwelling
occupancy
scaffolding
grants
cylinder type
solar system type

Grants currently in the UK are £400 and from 2011 there shall be a yearly grant introduced.

Average life-span of installations after 1996 are up to 20 [f]. Companies provide 25 year cylinder guarantees and 10 year guarantees on collectors mentioning life spans of over 30 years.

Sustainability-Environmental impact

It is possible to save roughly half a tonne carbon dioxide emissions.

Relevant European standards:

EN 806: General installation specifications.
EN 1717: Protection against pollution of potable water -device requirements concerning pollution by backflow.
EN 60335: Safety specifications regarding household and relative electrical appliances.
EN 94002

Solar space heating:

Alternative use of Solar thermal systems is space heating usually through connecting collectors to under floor systems due to the lower maximum temperatures required in this case (45ºC-50ºC). Especially when installed in concrete can prove to be an ideal heat sink and emitter.

Solar supported under floor heating systems require a cylinder size of more than 1000 lt. A considerable amount of solar panel area is required with an optimum inclination angle 60º-90º as they are focused in absorbing mainly winter solar radiation.

[k]

Stratified tank integrating solar collectors

[j]

This is achieved by roughly 10 m2 panels warming up the bottom of the thermal storage cylinder through heat exchange. A control system switches on the circulation pump when temperatures of the panels are higher than the cylinder bottom. A monitoring valve is responsible for circulating water under floor when return water temperatures remain cooler than those of the tank.

Requirements regarding storage tanks vary a lot. For 20-80% solar space heating autonomy a 0.8-5 m3 tank would be appropriate with size increasing up to 20m3 for higher percentages [l].

Although technically feasible, such systems have low specific solar gains and very high costs. They are never the less considered in Austria (well insulated dwellings) adequate for matching the whole heating demand.. However there are some issues concerning degradation of fluid in the panel (in summer due to overheating) and yearly low performance coefficients [l].

In order to reduce prices and boost the market companies have managed to present commercial more cost effective products combining storage tanks, pumps, controllers, improved tank efficiencies etc.

Some examples of stratification enhancement through new designs are illustrated bellow:

Photovoltaic solar electricity potential in European countries [l]

Some further improvements could be related to low flow operation through new pump models introduced, flexible tubes for easier installing and more sophisticated controllers presenting some diagnostic functions.

References

[a] Energysavers.gov - Solar water heaters
[b] En.Wikipedia.org - Collectors used in solar water heating systems
[c] R. Yao, K. Steemers/Energy and Buildings 37 (2005 663-671)
[d] En.Wikipedia.org - Evacuated tube collectors
[e] Europa.eu - Photovoltaic solar electricity potential in European countries (map)
[f] J.K. Kaldellis et al. / Renewable and Sustainable Energy Reviews 9 (2005) 499–520
[g] Greensystemsuk.com - On solar hot water
[h] Wisc.edu - Domestic Hot-Water Profiles in Different Time Scales
[i] Wikimedia.org - Collectors flatplate evactube image
[j] Ihsenergy.co.uk - Retro-fitting renewables in existing buildings
[k] Greensystemsuk.com - Solar assisted underfloor heating
[l] Solarenergy.ch - Solar thermal systems

Realistic Domestic Hot-Water Profiles in Different Time Scales Ulrike Jordan, Klaus Vajen solar@physik.uni-marburg.de

Solar panel Used:

Ozoneheatingsupplies.co.uk - Solar panels

Tank connected: 245 litre vented cylinder
Navitron.org.uk - vented cylinder

Total system with 245 lt tank:2000 sterlines with taxes
Navitron.org.uk - high spec solar kit