Hot Water Association member Ian Hughes of Kingspan Hot Water Systems looks at a common problem of solar-thermal installation: over-sizing.
It is all too easy to install a greater solar collector area in the home than is required. The reasons for this can vary, from customers trying to minimise fuel costs to planners attempting to achieve greater SAP rating on new-builds.
This practice can prove costly. Installing oversized solar collector areas increases the potential occurence of stagnation events.
Stagnation occurs when the storage volume reaches its maximum temperature and the solar loop cannot transfer any additional energy from the collector during times when there is high solar radiation. Stagnation causes the absorber to heat up to very high temperatures, which can cause irreparable damage caused to systems that are exposed to long periods of stagnation.
The whole system should be designed so that stagnation is eliminated or minimised as much as possible. You should choose solar cylinder sizes in the same way that you do for standard fossil fuel hot water cylinders.
The following steps outline how to correctly size a solar system in domestic hot water systems:
- Determine daily hot water demand
- Determine the auxiliary storage volume
- Size the solar volume
- Size the cylinder
- Size collector area
- Confirm cylinder and collector compliance
Determining daily hot water demand
There are two reference points for selecting the daily hot water demand. One of these is the government’s Standard Assessment Procedure (SAP), which was updated in 2010. In SAP calculations, the demand for hot water is derived from the floor area of the dwelling. The other reference point is BS 6700: Design, installation, testing and maintenance of services supplying water for domestic use within buildings and their cutilages Specification, which details volumes of water used per appliance. These figures should be assessed against the building design.
Determining the back-up storage volume
This should be calculated on the hot water requirement, taking into consideration the property design and draw-off-requirements. For example, in a 170m2 property with two baths, BS 6700 shows that minimum storage capacity would be 140 litres.
Using SAP to determine solar storage volume
The dedicated solar volume is the volume of water that can only be heated by the solar coil. The dedicated solar volume needs to be 30% of the total cylinder capacity.
The effective solar volume of a twin coil cylinder is the volume of the dedicated solar storage plus 0.3 times the volume of the remainder of the cylinder.
From the example, if the non-dedicated solar volume needs to be 140 litres, the cylinder needs to be 200 litre minimum capacity.
Size the cylinder
Choose the cylinder from the preferred manufacturer’s product ranges, selecting the next size available greater than the calculated values.
Refer to the manufacturer’s information to ensure auxiliary hot water volume is correct. Identify dedicated solar volume (Vs) from the literature.
An example would be a 250 litre cylinder with a dedicated solar volume of 75 litres and auxiliary volume of 175 litres.
Size the collector area
Capuring Solar Energy from the CIBSE Knowledge Series states: "The system should be designed to supply 100% of the building’s hot water demand during the summer months to gain full benefit from the installation". Ensuring that the system is not oversized at this point is the first step to provide protection from stagnation.
When sizing a solar system for domestic hot water, it is typical to size the system to achieve an annual solar fraction of 55%. The energy required to produce that amount of hot water is then calculated.
QHW = Volume of Daily DHW x Cw x (change in T)
Volume of hot water from data calculated
Cw = Specific heat capacity of water (1.16 Wh/kgK)
(Change in T) = Temperature difference between incoming and desired water temperature.
60°C – 10°C = 50°C
Continuing the example:
QHW = 159 x 1.16 x 50
QHW = 9.22kWh
Therefore the heat requirement = 9.22kWh/day
To size the required collector area the following formula can be used:
AR = (No. of days x QHW x solar fraction)/(Yearly solar irradiation x Av. system efficiency)
The average solar irradiation values can be supplied by reputable solar suppliers.
Continuing with the example for a property in Dublin:
Ar = (365 x 9.22 x 55)/(949 (Dublin) x 60) = 3.25 m2
The nearest size of collector would be 3m2
Confirm cylinder and collector compliance
From SAP a minimum of 25 litres per net square metre of solar collector storage volume is required; therefore, the example would require 75 litres of dedicated solar volume. Check this against the data provided by the cylinder manufacturer to confirm all requirements are met.
Original Source: Registered Gas Engineer Magazine – April 2011, ‘Solar Focus’ (page 35). Republished with kind permission from the magazine editor.
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