Introduction – copies to DaveC Keith Mr Bridges Tamsin Hi-Tech Energy Pat AndyP MartinS ebcec
FOE Birmingham had a 6M sq. Vacuum Tube solar hot water system fitted in Feb 2005. Previously provision of hot water was by instantaneous gas and electric point of use heaters.
The Solar installation included a large centralized hot water storage tank with immersion heating elements, and relevant new hot water pipe distribution. A spare heat exchange loop for future use with any possible gas space heating boiler was included in the storage tank.
System installed by Hi-Tech Energy Ltd, Burntwood, Staffordshire.
Key Performance Figures
Mar 2005 – Dec 2006
kWh produced by Solar (avg) 2866 kWh/yr [Actual] 4700 kWh/yr [Quote by Installer]
Solar Fraction (avg) 38%
Financial Performance
Cost of installation £10,500 (total)
Av Value of Energy displaced by Solar £387 (Electric) or £153 (Gas) [using 2866 kWh]
Payback periods 27yr 69 yr
Hot Water Provision
Hot water quantity/temperature/flow for Cafe and general warehouse use better than before.
Installation in General
The SE facing and unshaded location means the system runs for a good time when sunny and easterly element provides early heating ready for mid-morning use by Cafe. Overheating problems and melting of pipe insulation added after installation may indicate potential long term problem/wear/maintenance issue. Good standard of installation although no summary document provided with installation settings or maintenance schedule. Sagging of lower panel fittings may need rectification.
Tuning / Improvements
Delta T control setpoints seem a bit high compared to typical figures quoted in publications about SHW systems. Interaction with Immersion heaters an important issue especially as DHW consumption now appears to easily exceed the top half of the store volume. Serious overheating problems were not seen for summer 06. But was slightly dangerous for users.
Conclusions
The major use of Hotwater in the building, Cafe Dishwashing is currently not being supplied by the solar system. This is giving rise to a lower than expected total output from the Solar and also has highlighted the small size of the store. Recent energy price rises are in the payback times favour. Re-plumbing of Dishwasher would be financially beneficial and increase the contribution of the Solar. The system has functioned reliably with little intervention if any.
Ian Moore Nov 2006
General Points
SHW systems quoted generally as outputting 5 times as much energy in Summer as in Winter
Summer 05 had very low water use from the Cafe due to low customer turnover.
Historical changes to the Warehouse (items relevant to gas use too)
Feb 05 – Solar Hot Water system installation
Mar 05 – begins operation, Wash/mc added to DHW loop, Gas Water heater removed from Cafe Kitchen
Summer 05 – New owner for Cafe takes over
Winter 05 – Insulation placed on upstairs main bay roof
Spring 06 – Domestic Dishwasher replaced with commercial
Summer 06 – Dishwasher changed from 13A feed to 40A feed.
Installation Facts & Figures
Total Cost of installation : approx. £ 10,500 approx. 50% grant assisted by gov. clear skies
Panel Angle / orientation : 30 deg above horizontal / SEast
Panels : Schott ETC16 – 8 off Aperture 0.8m sq.
Quoted Thermal efficiency at 40 C above ambient is 0.7
Quoted Thermal output is 730 kWh/yr meter sq. (in Germany)
Storage tank : 275 litres, 64x20 (1.625m x 0.51m) Insulation Jacket ?
Circulation pump : Grundfos UPS 15-50 (consumption 40-95W)
Controller : RESOL DeltaSolB
Finance - Estimated
Current cost of Electricity : 10p / kWh (Gas is 2p/kWh)
Estimated heat output of Solar System = approx 5000 kWh/yr
Saving for displacement of 5000 kWh/yr of Electricity = 5000 * 0.10 = £500
Saving for displacement of 5000 kWh/yr of Gas = 5000 * 1.33 * 0.02 = £133
(1.33 is to allow for extra gas use, boilers are not 100% effic. unlike electric immersions)
System Performance
Heating water by the Sun quotes typical pumped water flow rate of 0.015 kg/s/m sq.collector.
= 0.015 * 60 *60 * 6 = 324 litres per hour
Solar Installations quotes 0.4-0.6 litres/minute/m sq. collector ?in series
= 0.5 * 60 * 6 = 180 litres/hour
Assume the flowmeter reading of 20 is in lbs per min, this being constant under all operating conditions through the year. (the units are not marked)
20 lb/sec = 20/2.2 x 60 = 545 litres/hr
Readings taken during operation by Keith suggest that during operation the temperature difference between the heat exchange fluid entering and leaving the tank is around 5 deg. C for winter and 8 deg. C for summer. (This would correlate with the Sun being 2 to 3 times as strong in the summer)
The circulation pump is of the constant speed type.
Hence the thermal output of the Solar system to the storage tank is given by
Heat Transfer to tank = flow rate * SHC * temp. diff.
For 1 hr of pump oeration
= 545 * 4.186 * 8 = 18251 kJ
= 18251/3600 = 5.07 kW
The above figure uses 8 degrees as a temperature difference for the heat exchange fluid.
Fig.1 – Immersion Electric kWh against scaled Solar pump run hours (March 05 - Oct06 )
(top line is combined solar & immersion)
Figure 1. uses a scaling factor to indicate heat output from the solar system based on the 545 litres per hour flow rate and a working HX loop temp diff of 8 degrees for May/June/July and 7/6/5/4/3 degrees respectively for the for the other months in the year.
Solar Energy Output
2005 - 3232 kWh
2006 - 2500 kWh (assuming Nov and Dec only produce around 50kWh)
The 2005 figure maybe an overestimate due to an apparently large number of pumping hours in early spring 2005 when it was being commissioned. August 2006 was an unusually dull month and hence its normally large contribution was absent.
Immersion Heater Readings
Another way of estimating the (useful!?) output of the Solar system is to look at the Electric Immersion meter readings during winter. Presumably with constant hot water use throughout the year (assumes constant cold water inlet temp.) then reduction in Electric Immersion heater readings should be as a direct result of the heating provided by the Solar.
Complications regarding setting of Immersion Heater Timers may cloud the results but if the above presumption is true then during the peak of summer the Immersion Electric should reduce to almost zero and the Solar output should climb to about 25 kWh/day. (Peak winter consumption of Immersion Electric was about 25 kWh/day in Winter 05)
Points to note :
Winter operation will presumably have more stop/start operation so pump hours recorded may not be as accurate in this season. With Toff at a possibly high 5C the system may be operating in a sort of 'batch' heating mode for many months?!
Typically solar systems gain most of their yearly proportion (80%) of energy from the Summer months. Hence pump run hours for winter with low HX fluid temp. diffs. Will cumulatively add up to a small amount of Energy.
Solar Hot water produced may be more beneficially used at certain times of the day and also effect the operation of Electric Immersion heaters used in the hot water store.
Peak Output of panel system should be approximately
panel aperture * no of panels * efficiency * Peak Solar Radiation for location
* 8 * 0.8 * 1.0 = 5.12 kW
This value would appear to be in keeping with expected HX temperature differences during height of the Summer (8 deg C).
Manufacturer quotes 4672 kWh/yr (0.8 * 8 * 730) as the annual output. See Schott brochure !
System Settings
No record of installed system settings seems to have been supplied on installation.
For Domestic sized systems (as opposed to District) with conventional flow rate collectors a number of publications quote Delta T contol settings for normal operation to be
Publisher
Ton dif. 4 C Toff dif 1.5 C B.S 5918
5 C 2 C Lars Andren
4 C 1 C Heating Water by the Sun
Delta 6 C How To Capture a Little Sunshine
Volume of collectors compared to pipe run volumes and HX
Difference between summer peak op and winter
Solar insolation for winter less than half that of summer? hence controller will cycle if toff is 5C.
Fit in pipe sensors?
Basic Solar theory
Site specifics sun diagram etc
Performance Indicators
SHW systems quote amongst other things Solar Fraction – percentage of annual DHW supplied by Solar
Solar Fraction = Solar Energy Contribution / Total Energy Consumption for 12 months
for 2005
= 3232 / [ 2793 + estimate for Jan+Feb 2005 of 1300 ]+[3232]
= 3232 / 7325 = 44%
for 2006
= 2455+estimate for NovDec of 45 / [3975 + estimate immersion NovDec of 1100] + 2500
= 2500 / 7575 = 33%
Although the Cafe Dishwasher is fed by the cold mains it is interesting to work out the Solar fraction including this figure to give a Solar Fraction for the 'Overall Building' rather than just the Immersion/solar store fed services.
for 2006 incl Dishwasher
= 2500/ (7575 +2500) 17 cycles for 300 days at 0.5kWh cycle
= 25%
Store Size
Store sizes have a large influence on the expected annual contribution a Solar system makes and hence its Solar Fraction.
Typically stores are sized so that they have a dedicated lower section for the Solar input. i.e The top part can supply a full days quantity of hot water without need to use auxilary heat on the lower part. A lower store ratio of 80% of the daily Hot Water use is quoted in How to capture a Little Sunshine as being a minimum size to use if all solar production is to be utilised.
Heating of the lower part of the store reserved for the Solar input will reduce its capacity to accept heat from the solar input.
Typical DHW Solar systems are quoted as needing 50litres storage and 1m sq. of panel per person (for a typical use of 25 litre DHW per person)..
i.e a 4 bed house would use a solar system with 200 litre tank and 4m sq. collectors. Assuming these are conventional flate plate collectors the FOE system is probably equivalent to a 9m sq. flat plate array. Hence a 450 litre tank would be appropriate.!? And should give a standard 40 to 50 % solar fraction.
BS quotes pre-mid morning and after sunset DHW consumption as badly affecting Solar output when a store size is small. Presumably careful use in daytime may have advantage of reducing auxillary heat input when using a small tank?!
Calculation for store heat up time.
For 100 kg i.e 100litres heat from 10 to 60 C (SHC water 4.2kJ/kg/C)
heat = 4.2 x 100 x 50 / 3600 = 5.8 kWh
for 200 litres of water = 11.6 kWh
For a 400 litre tank
The top half, 4hrs with the single top 3kW element
The whole tank with top and bottom 3kW elements 4hrs
Bottom half with peak sunny output from solar of 4.5kW 2.5 to 3hrs.
This last figure suggests possibly there are too many panels on the roof for the store. ?!
5-6hrs to heat a whole tank without auxilary immersions.
In reality stratification in the tank will mean that the bottom part of the tanks heat will eventually rise and the upper halves cold will eventually fall.(to good effect)
Building Water usage per day
Winter immersion consumption of an average of 25 kWh/day suggest a DHW consumption of :-
25 x 0.75 (suggested factor for heat losses!) / 5.8 = 323 litres
Check with Cafe !
See Fig. 12 in Heating Water by the Sun for effect of too many collectors.
Location of Sensors
Sensor S3 installed in the pocket near the top of the tank (shown as TT on the controller display) simply displays the temp near the top of the tank. The thermostat function which controls the spare unconnected relay on the controller is unused (typically functions as boiler turn 'on' or surplus heat use).
Sensor S1 is installed on the insulated pipe exiting the 'hot' side of the collector array.
Sensor S2 is installed on the insulated return pipe to the collectors just as it leaves the storage tank. The HX loop appears to be connected in the conventional way of 'hot' fluid entering at the top of the coil and the 'cooled' fluid exiting at the bottom.
Hence transfer of heat from the collectors to the store should be easily and accurately achieved by the Delta T control implemented by the Controller. (Although the sensor S2 is not specified by RESOL as to be placed in this location)
Overheating
The Controller also uses S2 as 'store temperature'. [See RESOL Manual - 2.Sensor types, 6.1Examples)
Current location of S2 will may provide poorer overheating protection for the tank or at the least a reducing adjustment to SX will need to be made.
Observed temps for 5th June 06 2.30pm
TC = 82C TT = 78C TS = 72C SX = 75C
RESOL manual section 2.Sensor types states that for stores with 'integral heat exchangers' the sensor 'must be mounted in the upper part of the heat exchanger'. Looking at typical system drawings on the internet and in publications this is implemented by having a sensor pocket in the tank in the middle of the lower heat exchanger area or just at the top of it.
System Monitoring
There are a number of options if improved monitoring/evaluation of performance is required.
Place or combination of below :-
Water meter on the hot water header tank (£40)
Data logger on the already existing controller (£100)
Fancy Resol meter for combining both of above measurements (?)
Check panel balance by measuring Temp outputs on Panels. does this actually matter?
Suggestions/Comments
Possibly move repeater display to near Cafe kitchen or certainly to a busier location than the Resource room. Cafe is main user of water so may be interested in keeping an eye on the temperature in the tank (with a view to possibly switching on/overiding the immersion the themselves, alternatively put in reception or the downstairs kitchen).
Setting of immersion heater timers likely will need different 'programs' for winter/summer as well as individual day programs and possibly manual overides. Small size of the water store makes these immersion settings more significant to the contribution the solar makes yearly and maximisation of collector output use.
Enquire as to price of off-peak electric tarrifs.
May be interesting to work out if the Solar System has paid for itself yet in terms of Energy required to manufacture and install.
Fit hinged or lifting door to storage tank area for easier access. Measure lower sensor pocket temp
Location of pressure relief valves may be unconventional.
Cooling of system has been provided previously by Keith running off a bit of HotWater.
Could use spare Thermostat function output relay.
Notes
Since the solar controller has been set with a store limit of 75C likely there has been production of hot water when not required. Hence the output of the Solar system will be slightly above the 'useable' output. High panel/store/pipe temperatures always result in much greater losses to ambient. Maybe measure mains cold water inlet temp this winter for interest.
Finances
The Cafe would have stopped using a large proportion of gas heating anyway due to new dishwasher. So utility bills would still of seen a large rise due to transfer from gas to electric for general dishwashing.
Value of conventional fuel displaced
Average yearly output for 2005 to 2006 is (3232 + 2500)/2 = 2866 kWh
Electricity : using 13.5 p/kWh (Autumn 2006 price) = 2866 x 0.135 = £387
Gas: using 4p/kWh (this is a guess) = 2866 x 0.04 x (1/0.75) = £153
0.75 is assumed conversion efficiency of gas water heater
Payback period (simple)
= 10,500 / 387 = 27 yrs for Electric
= 10,500 / 153 = 69 yrs for Gas
Maintenance costs are not included.
These payback period figures are significantly more favourable than if 2004/5 utility prices were used. Note also that due to the relatively large capital cost of the installation (like most RE systems) the maximisation of the usable output is crucial to giving an attractive payback period. Obviously the format of existing heating systems, planned replacements and integration of any future changes of appliances or heating related systems is crucial.
Information on how the installation has effected general electricity consumption in the building can be found in the Electricity Survey undertaken in 2006 at FOE Alison St (see References)
Setpoints
Setpoints June 2006
Controller
DO 10 CX 120 FN 1
DF 5 CN 20 MM na
SX 75 TO 40 PG 53.25
CL 140 TF 40 VN 1.01
Top 3kW thermostat
53 C
Immersion Timers
Top
Bottom
References
Electric Survey - http://iansengineeringpage.org.uk/electric_survey.html
Photographs of installation - http://iansengineeringpage.org.uk/solar_thermal.html
Abbreviations
DHW – Domestic Hot Water
HX – Heat Exchanger
RE – Renewable Energy
SHC – Specific Heat Capacity
SHW – Solar Hot Water System
Publications
Tapping the Sun – FOE Library
Heating Water by the Sun – FOE Library
BS 5918 Code of Practice – Solar Hot Water Installations - Birmingham Central Library
Solar installations – Lars Andren
How to Capture a Little Sunshine – FOE Library
RESOL DeltaSolB – Manual available on Web at http://www.resol.de
Useful Info
SHC Water = 4.186 kJ / kg C
1 kWh = 3600 kJ
Peak Solar Radiation for Latitude 50 North inclined surface of 30deg estimated to be 1000W/m sq.
Suppliers of high temp pipe (to 150C) – Armacell UK , ref Worcester Bosch website
Explanation of Controller – RESOL DeltaSolB,
Hardware Sensors and connections
Collector – Solar Collector Panel mounted on roof
TC – Temperature of Collector (presumably the water inside at the exit side of the panel)
TS – Temperature of Hot Water Storage tank. (Not the top of the tank but !hopefully measuring the temperature of the water in the tank just above the HX loop?this may be very similar to the temp. of the hx fluid leaving the hx, this exit pipe typically being where delta T controllers locate the second sensor)
Issue with control of overheating of tank and measurement of delta T of hx fluid leaving store?
CL – Collector Limiting, pump turns off “to avoid damaging overheating of the solar components (collector safety shutdown)”
TT – Temperature for use by Thermostat function (not used at FOE installation). Sensor is inserted in top of the tank for display purposes.
DO, DF – Implements simple delta T control. Pump will turn on if Collector is sufficiently higher in temperature than the store. Pump will turn off if Collector temp is very near Store temperature.
SX – Storage temperature max. Also used as a set point for pump turn-off when the system has been in cooling mode and the tank was above this temperature (i.e it was taking heat from the panels).See FN2/3
User Selectable Operation Modes
When in automatic mode the controller can respond to increasing levels of system temperature in 4 different ways.
FN 0 - “Maximum Store Temp De-activated”
If the DeltaT setting is true, then the pump will circulate and try and raise the temperature of the store.
Except for below conditions :-
Pump off if TS > 90C 'store safety limit' OR TC > CL collector limiting temperature reached
FN 1 - “Maximum Store Temp Activated”
Described as 'pure maximum temp limitation',
If the DeltaT setting is true, then the pump will circulate and try and raise the temperature of the store up to the setpoint SX. (note max 85C)
Except for below condition :-
Pump off if TC > CL collector limiting temperature
FN 2 - “Max Store Temp de-activated, re-cooling function activated”
'Avoids overheating of the collector'
This mode will continue running the pump past the SX setpoint and into the evening(or cloudy weather) until the collector/system starts to act as a radiator of heat. When TS returns below SX the pump switches off.
If the DeltaT setting is true, then the pump will circulate and try and raise the temperature of the store. If the pump is running, TS has been exceeded and the DF condition is true the pump will remain on until TS returns below SX.
Also Pump off if TS > 90C 'store safety limit' OR TC > CL collector limiting temperature reached
FN 3 - “Max Store temp Activated, re-cooling Func Activated”
Controls maximum temperature of store as in FN1 but also allows cooling of collector if necessary.
If the DeltaT setting is true, then the pump will circulate and try and raise the temperature of the store up to the setpoint SX. (note max 85C)
If TC reaches CX then pump will circulate until TC falls below CX. IF this cooling has taken the store temperature above SX and the collector is cooler than the Store then the pump will operate to bring the store temp down to SX.
Also pump off if TS > 90C 'store safety limit' OR TC > CL collector limiting temperature reached
Notes for all modes
– Pumping never takes place when TS is above the 'store safety limit' of 90C
Analogy for modes – hopefully simpler than above
FN0 – Transfers heat from collector to store subject to 90C limit
FN1 - Transfers heat from collector to store subject to SX limit
FN2 - Transfers heat from collector to store subject to 90C limit, but if pump is running and store is above SX then the pump remains on until excess store heat is transferred back to the collectors and TS returns to below SX
FN3 - Transfers heat from collector to store subject to SX limit, but will restart and cool the collector if CX reached, afterwards when the collector has dropped below store temp, will dump the now excess stored heat back out through the collector.