Optimising solar water systems
Although the choice of collector for solar domestic water heating (SDHW) will significantly affect the annual performance of a given system, it is often overlooked that there are also important gains to be made by optimising the rest of the system. The Northern European climate will easily allow 50% of annual average DHW (the Solar Fraction) and fractions of 76%1 have been recorded in the UK. However if boundaries are to be further pushed back, then the complete system design must be considered. This includes storage, heat transmission and pump control. Research and simulation data, whilst available for collectors, is lacking in these other areas.
Of central importance is the storage method. This is typically a copper or steel
vessel, usually located remote from the collector in the living area. The store
in effect matches the varying solar heat source to our equally daily demand
for hot water. The important performance factors here are vessel insulation,
volume and stratification 2. A high level of insulation to
store walls is clearly desirable, a minimum of 40 mm if rigid foam and better
still 80 or 100mm. What is often forgotten is that heat loss can occur via immersion
heater heads, uncovered tappings or even the adjoining pipe work.
Stratification of the store is desirable to increase heat transfer efficiencies
both at the collector and store, especially as much of the solar gains are made
via temperatures lower than that of the target DHW demand of say 60 C. For a
given volume of water, a tall thin store will promote a greater difference between
the top and bottom hence leading to positioning the solar transfer coil in the
cool, lower portion3. Some more sophisticated stores avoid
coils and ensure that the highest temperatures go to the top without being diluted.
A large store is useful to absorb the summer peak gains, however if this can becomes too big with an unwanted greater call for summer fossil fuel top-up, which may have been needless if the store had been better sized. The usual advised range of ratios of stored secondary water to collector area varies from 30 to 60 litres per m2 depending on expected peak solar insolation (solar intensity), the consumer's DHW profile, and collector performance. The calculation of this ratio could be made more precise by using validated collector test data and local insolation (solar intensity) figures however a bias towards a larger store will benefit the durability of the system due to decreased temperature differentials i.e. metal fatigue and antifreeze degradation.
Although there are no medical cases known to the author, to connect solar stored DHW water with bacterial health risks, it is sensible to ensure (as with all DHW production and storage) that a risk assessment is made. 4 Good system designs will ensure that if the secondary DHW is being used as the thermal storage medium (typical for most domestic applications) then an auxiliary energy source is fitted. This source is frequently an upper boiler heated coil, normally located at the top of the store, combined with electric immersion elements set on appropriate timers ensure pasteurisation at the required intervals. Where a high-risk assessment is made, the alternative is typically to isolate the storage medium from the DHW by use of a thermal store, a third output coil or even a pumped plate exchanger. Alternatives to the removal of store stratification are normally sought due to its importance to collector efficiency. It is generally considered in the EU solar trade, that maintaining a constant hot 'top' to the store by a fossil fuel back up is the primary method to address bacterial health risks.
Many end-users delight in each degree achieved as the sun rises each
morning and enjoy the connection of their lives to that of such a powerful natural
energy source. However, it should not be forgotten that some solar thermal collectors
shouldn't be simply shut off when a store becomes too hot, as without a moving
transfer fluid to extract heat during high solar gains, dry internal collector
temperatures may exceed 200C.. Recent collector types are now designed to automatically
accommodate this either by drainback or special transfer fluids. Overheating
of secondary water by a solar system, (i.e. above 70 C.) whilst impressive to
the novice, is a problem not only for durability but perhaps a risk of scalding.
Extra equipment such as thermostatic mixing valves and diverting heat sink circuits
are frequently used to control some systems.
An interesting development is with the use of external plate heat exchangers, as opposed to a coil attached to the store. Whilst requiring an extra pump, they do allow higher transfer efficiencies and ease of maintenance. For aggressive water types this maintenance is essential to maintain reasonable heat transfer.
Integration with a fossil fuel back up for DHW comfort and safety is a virtual necessity during the shortest days of the year in Northern Europe. In the UK particularly, there are a myriad of pipe, valve and store configurations available to integrate the solar system that include single stores with multiple coils through to multiple stores through to 'instant' top-up appliances such as combi-boilers. Whatever is chosen, the quality of control of the back-up heat source is the key to minimise unnecessary fuel bills and careful use of time and temperature switches are a pre-requisite. It underpins any solar thermal system that the back up fuel is only brought in when necessary.
Where the solar primary circuit requires a pump, a differential
temperature controller (DTC) is usually required. In its simplest form, a comparison
between collector (high sensor) and store temperature (low sensor) is made (the
differential) to enable solar heat to be given to the store without needless
electric use or wasteful dispersal of stored energy. An overlooked heat loss
comes from the pump casing, which has to be ventilated for many pump circulators.
However some manufacturers now allow the whole pump to be covered. Around 75%
of electrical parasitic energy driving the pump is absorbed in the transmission
fluid although this can be increased to 94% with case insulation .5
There is much to be said for a direct photovoltaic-driven low-voltage DC pump
which not only ceases parasitic electric losses but automatically modulates
the pump speed to match the solar insolation. However a true temperature control
will allow the collector to perform at its most efficient and some systems no
combine the best of both worlds.
The DTC relies on the correct temperature information to succeed,
as well as appropriate setting of differential and hysterisis (or switch delay).
A high differential, above 7 C. between collector and store, is appropriate
where system losses in pipework and transfer are high. The hysterisis (the difference
between switching on rise versus fall) will prevent short cycling of the pump
switch. The high sensor should be located so as to represent the temperature
of the collected solar energy. This means that it will be mounted adjacent to
a collector waterway or in a collector manifold pocket and measuring the temperature
of the body of primary water sitting in the collector. A poorly positioned sensor
would be mounted on the surface of a collector flow pipe as there is a risk
of missed pump opportunities as clouds pass by, due to a large delay in the
sensor response. Conversely, a sensor mounted far from waterways of a collector
surface may give false high temperatures during sunbursts. Preferred locations
are dependent on water content of a collector, for high water contents an immersion
sensor is fitted in the waterways of the collector whilst for those with low
water content and high active area a surface mounted sensors is used. In all
cases the backs of the sensors should be insulated. Similarly for the low sensor,
the sensor should represent the temperature of the secondary water soon, to
receive the pumped heat (via the transfer surface if present). A poor location
would be upon an uninsulated return primary pipe far from the cylinder that
will cool rapidly. Far better to be immersed inside the store itself. The choice
of heat transmission fluid has some effect on the system design e.g. where the
lower specific heat capacity of polypropylene antifreeze may in some instances
raise the operating temperature causing greater collector and transmission losses. 
By use of pulsed width modulation of an AC electric waveform, it is now possible for the DTC to vary the speed of an ordinary AC pump as temperatures vary across the collector, hence boosting collector efficiency. Extra bundled monitoring equipment within the DTC, whilst not directly improving the system performance, do allow easier supervision and comparisons to be made, especially those which calculate and log energy gains over many months.
An optimum demand profile is from a household, which uses solar generated DHW as soon as it reaches a (preferably) low target temperature, usually late morning onwards in summer, hence effectively creating more cool secondary storage. A bad profile would be peak demand of high temperature water early in the morning. Since few households are able (or desire) to change their lifestyle in this way, this may not have much meaning however it is worth noting that certain system designs are better suited to some demand profiles i.e. the greater the time difference between solar energy supply and DHW usage, the more important the quality of storage becomes. Theoretical computer models can allow dedicated users to easily anticipate DHW regime changes to best benefit from solar energy.6. In the ETSU study1 , it was interesting to note differences of DHW monthly consumption of up to 52% less than a formula typically used by British Gas.
Solar for space heating is less common in the UK. The necessary increase in collector area compared to DHW-only generation can cause problems with summer over-sizing. This is overcomable with good design and a number of off-the-shelf systems exist. The principal requirements are fast responding low temperature heat emitters and a means to transfer low grade heat at the coolest point in the circuit, usually at the boiler return. An alternative technique is the super thermal store, where many forms of low and high grade heat can be combined for selective take-off with high flow rates of domestic hot water. These have the potential to provide the ultimate solar fractions i.e. + 80% although the space heating component rarely exceeds 20% and can also be used as the basis of heat saving strategies in larger buildings.
FOOTNOTES
1. ETSU Comparative study S/P3/00275/REPAnalysis of four active solar installations
2. Results From Testing Of Small Heat Stores For Domestic Hot
water And Space Heating
Author(s): Peter Kovács
Martin Sandberg
Swedish National Testing and Research Institute
P.O Box 857
501 15 Boras
Sweden
3. Numerical Study of Flow and Heat Transfer Characteristics
in Hot Water Stores
Author(s): Y. Chen
E. Hahne
Universität Stuttgart, Pfaffenwaldring 6
70550 Stuttgart, Germany
Optimum Selection for Aspect Ratio of Solar Storage Tank
Author(s): K.K. Matrawy
I. Farkas, J. Buzas
Gödöllö University of Agricultural Sciences, Department of Physics
and Process Control
Páter K. u. 1., H-2103 Goedoelloe, Hungary
4. HSE Books 2000 ACOP&G L8 The control of legionella bacteria in water systems.
Introduction | Getting into Hot Water with the Sun | Optimizing Solar Water Systems |
Copyright © 2002 . All Rights Reserved.