EMC Testing – my experience

ed1066

Member
I started a Thread on 9 Apr 2013 “EMC Testing” before attempting to get my control circuit based on a 20X2 through the UK EMC tests. People gave me helpful time and advice and I’m reporting back here after completion of the testing.

Before I get going, I’ve tried to see if there has been any discussion in between but the Forum search facility failed me. Any ideas why? I clicked ‘Search Forum’ then entered “EMC” kept the ‘Show Threads’ option, clicked ‘Search’ but got “Sorry - no matches”. That’s despite my earlier thread with “EMC” in the title. (Searching “testing” produced pages of threads)

Anyway, the controller is for controlling the 12V pump in a solar thermal system. After 4 half-day visits to my test facility, I finally had a pass. There were three areas requiring work:

Use of pwm
Cables to DS18B20 temperature probes and separate OLED/LCD display
Vulnerability to electric shocks in supply and signal leads

I will break this up by posting on each after this initial post.

Firstly - The Original Circuit

Here is my original circuit, somewhat simplified (eg no download cable, non-relevant input removed and only one of three DS18B20 sensors shown).

circ diagr2.jpg

Some notes:

  1. The control circuit PCB is in a plastic box.
  2. The pump has a brushless motor, it does not produce back emf. Following the April Forum discussion I removed its motor capacitor. The lead from the control box to the motor is a 20cm length of simple two-core speaker cable.
  3. About 4m of the same speaker cable powers the system from a 12V switched mode power supply.
  4. There are three DS18B20 temperature sensors, on 2m – 6m or so leads, simple 4 core ‘alarm cable’ with one core not connected to anything (3 core not available). One sensor shares a lead with the OLED for some of the way. One capacitor covered all the sensors (the OLED has its own)
  5. The 27 ohm resistor covered power to all the temperature sensors & OLED and was intended to protect the circuit if a short occurs in a lead or connection.
  6. The OLED is the AXE133Y, its lead is 3 – 5m long and its in a plastic box.
  7. The frequency is reduced to 4 MHz which is that used by the ReadTemp command. As I need ReadTemp that seemed to be the lowest frequency possible.
 

ed1066

Member
Emissions and pwm

The first tests showed the motor was associated with noisy radiated emissions above the permitted limit.

· The problem only arose when the power to the pump was being reduced using pwmout.

· Using a linear power supply rather than a switched mode power supply actually made it worse

· Adding ferrites on the power line to the pump did not help enough

· Conducted Emissions were not a problem

The problem was solved by adding a 1K resistor to the connection from the output pin to the FET. I tried 2.2K as well and 1K was sufficient to reduce noise with only a slight increase in temperature (0.5 deg C) recorded on the FET heat sink, and a slight change in total power consumed. The 2.2K caused a higher temperature rise and a higher power consumption.

See the updated circuit diagram with this and the other changes.

circ diagr3.jpg
 

ed1066

Member
Immunity - Cables to DS18B20 probes and OLED

Any cables longer than 3m needed testing, which confirmed there were problems with both the leads to the temperature probes and the OLED. Noise added to the leads with a heavy coil thingy knocked out the signals.

The book recommended to me by this Forum back in April was helpful. It’s by Tim Williams: “EMC for Product Designers”. The Fig 13.16 on p358, for example, suggests that capacitors are needed due to the high impedance at each end of the signal cables.

I did tests with a piezo gas lighter connected to a circuit to produce transient currents – see photo.

Testing_cr.jpg

It showed the vulnerability was reduced by a combination of resistors on the signal and capacitors to ground. The capacitor should be on the cable side, the resistor nearer the pin on the 20X2. The values for these components were eventually derived using trial and error (hence several visits to the EMC test facility). For the temperature probes with more than 3m cables a filter is needed at the PCB end only. The resistor is 330ohms and the capacitor 2.2nF.

For the OLED signal cable a filter is needed at the OLED end only. The resistor is 1K and the capacitor 10nF. The 5V supply also needed a 12 ohm resistor at the OLED end, to prevent loss of voltage I reduced the 27 ohm resistor beside the voltage regulator to 12 ohm.

I tried twisted wire pairs instead of simple alarm cable, but it was marginally worse with twisted pairs.

I added a code signal, the number 199, to the OLED prior to any other signal. If serin in the code controlling the OLED read a byte variable equal to 199 then it went on to read the data to display. So if the signal was corrupted, the display didn’t update rather than displaying random spurious messages.

The system only just passed these tests, I was told that to get better results the only option would be shielded cable and enclosures etc. For the final circuit, the vulnerability was greatest at the high frequency end, eg 30 to 80 MHz.
 

ed1066

Member
Susceptibility to Voltage Shocks

In the EMC tests, this is the items ‘Fast Transient Immunity’ and ‘Surges’. The shocks were applied to the 240V supply to the switched mode power supply and to the signal cables (to DS18B20 probes and OLED)

The shocks knocked out the controller and it restarted after the test. This was a pass, but it showed vulnerability and I was strongly advised to address it.

Firstly, I worked on the circuit layout on my PCB, based on the Tim Williams book. I made it into 3 zones: (a) 12V supply, the voltage regulator and pump power control, (b) interface for temperature & OLED leads and (c) the inner protected circuit around the 20X2 chip. Each zone had its solid ground plane on the underside of the PCB and they were connected together at one central point only. All the signals & 12V/5V lines were on the upper plane of the PCB.

I then added a 220ohm resistor in the 5V supply to the 20X2 chip. Tests with the piezo gas lighter showed the vulnerability was resolved without problems in voltage to the chip.
 

srnet

Senior Member
Very Interesting, what was the cost of the testing and advice ?

Just a general idea would be useful.
 
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MartinM57

Moderator
Yes, makes the the forum's annual subscription more than worthwhile :D

Well done - in achieving it, and documenting it
 
Very interesting indeed ... thanks or posting this.

As a newcomer to the forum, I hope I have this right:
If you want to search for a three-letter "word" you need to add a wildcard asterisk
i.e. search for EMC* instead of EMC

MichaelG.

Edit: Strangely ... that doesn't find your threads :-(
 
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ed1066

Member
Thanks to All - - searching

Thanks to all those leaving feedback, it really made it worthwhile doing the write-up.

The only issue is whether anyone facing EMC in the months ahead will find the thread

...................you need to add a wildcard asterisk
i.e. search for EMC* instead of EMC
.....................
..... Strangely ... that doesn't find your threads :-(
I also tried this with the same result
 

eclectic

Moderator
Thanks to all those leaving feedback, it really made it worthwhile doing the write-up.

The only issue is whether anyone facing EMC in the months ahead will find the thread



I also tried this with the same result
Try the test as in the screen-shot.

Any luck?

e
 

Attachments

lbenson

Senior Member
You might put "electromagnetic" somewhere in your post so that anyone who thought to search for that would find it. Does the "C" stand for "Compliance"? Electromagnetic Compliance then?
 

ed1066

Member
You might put "electromagnetic" somewhere in your post so that anyone who thought to search for that would find it. Does the "C" stand for "Compliance"? Electromagnetic Compliance then?
Thanks - I've edited the tags to include "electromagnetic".

Further oddities - I only saw your question on the C in EMC after clicking "Reply With Quote". C is "Compatibility" and I've added that too.
 

eclectic

Moderator
Yes "EMC Testing" worked, but keeping to the screen-shot settings and using text "EMC*" didn't. Strange
To raise the chances in the future,
Re-post your experiences in the

Finished Projects Miscellaneous area.

Include links to your other threads while you're there.

e
 

Technical

Technical Support
Staff member
In general the vBulletin forrum search engine does not cache 2 or 3 letter words, there are just too many of them (of, it, in, and, the, not etc. etc.) and the search then takes far too long.
However if you combine with another word as Ec shows above it should then work.
 
In general the vBulletin forrum search engine does not cache 2 or 3 letter words, there are just too many of them (of, it, in, and, the, not etc. etc.) and the search then takes far too long.
However if you combine with another word as Ec shows above it should then work.
For my education ... could you please explain why a search for EMC* does return some results
[I think there were eleven, when I tried]

Much appreciated.
MichaelG.
 

hippy

Technical Support
Staff member
For my education ... could you please explain why a search for EMC* does return some results
That's because the "EMC" string is included in those threads. In the "Retrieve Program From PICAXE" thread it was started by member "emcp", in the "O-Silly-Scope" thread there's a reference to "Emco Electronics" etc.
 

lbenson

Senior Member
In my text-search engine writing days, over 30 years ago, we had not only limits on the length of words to be searched for (also 3 characters as I recall), but also "stop" lists (which would include words like "this", "that", and "which") and "include" lists, which could have entries like "EMC" (but not, in this case, "LED", because that would generate too many hits).

An "include" list would have to be maintained by someone, and new words added as the need became apparent.

It might be useful if the "xxx*" search would match just "xxx" as well as longer terms.
 
Thanks for the explanation, Hippy ... although it does make this Search Facility rather unusual.
My understanding was always as described in this Wikipedia entry i.e. that the star meant "none or any".
As Ibenson suggests, it would be useful if it could be made so.

MichaelG.
 

hippy

Technical Support
Staff member
My understanding was always as described in this Wikipedia entry i.e. that the star meant "none or any".
Wildcard pattern matches are always a matter of implementation choice but I would agree that seems a reasonable interpretation and expectation.

From the vBulletin message when trying to do three letter searches - "For 3 letter words add * (e.g. i2c* )" - it would seem that should apply although the search results do not seem to reflect that.

It may be an issue which needs to be addressed by vBulletin.
 

premelec

Senior Member
This result on the DS18B20 using long leads would lead me to think using a thermistor could be a better choice for passing the tests despite non-linear correction complication [lookup tables]. Thermistors can be robust for over temperture as well. Good luck with the product! [BTW in past years I have found several installations of solar hot water panels where their thermistor lead wires had broken from mechanical [e.g. wind] problems - so tie 'em down!].
 

ed1066

Member
This result on the DS18B20 using long leads would lead me to think using a thermistor could be a better choice for passing the tests despite non-linear correction complication [lookup tables]. Thermistors can be robust for over temperture as well. Good luck with the product! [BTW in past years I have found several installations of solar hot water panels where their thermistor lead wires had broken from mechanical [e.g. wind] problems - so tie 'em down!].
Thanks premelec. I did have a go with PT1000's because they seem to be the standard for solar hot water. I had the help of a friend more conversant in electronics than me. It became very complicated very early and I gave up. The problem is partly because I want to know a difference in temperature quite accurately so about 0.125 deg C accuracy is important, (for DS18B20's with readtemp12 its 0.0625 C)

Great steer on lead wires & wind - thanks. Any other tit-bits?

Cheers, Ed
 

premelec

Senior Member
@ed1066 - It will be hard to get the resolution you hope for whatever sense unit you are using since there are many mechanical and time factors involved - slightly different mounting, lead wire lengths response times and thermal coupling etc... I suppose you are looking for "steady state constant flow" sorts of BTU gain and such. Calorimetry is an ancient art - investigated by the like of Lord Kelvin before micros... :) The problem with things like PT1000s is the resistance of the lead wire is likely a bigger factor - with copper having high positive tempco and constantan not being available ... current devices like the AD590 avoid lead wire resistance effects but may not track so well. Anyhow there are tomes written on temperature measurements... One of my favorite solar multiple Dallas one wire temp references is the arbiter at www_anotherurl_com which he made many years ago for a solar system. You could use very low power wireless connections that probably would pass RF radiation tests... etc - likely you've considered all that. One last comment.... pay attention to where the solder is going when you are feeding it into a pipe joint - I once saw a guy put about .5# solder into a joint that dropped down into the pipe a way restricting the flow... :-0 Not too hot, not too cold - just right! Anyhow good luck with it...
 
@ed1066 - .... The problem with things like PT1000s is the resistance of the lead wire is likely a bigger factor - with copper having high positive tempco and constantan not being available ...
I was rather surprised to read this, because [when I worked in an Environmental Test lab.] we used to weld-up our own Copper/Contantan thermocouples to whatever length we needed, using wire from the reel.
A quick check here confirms that Constantan wire is available in the UK.

Hope that helps [it's about all I can offer project-wise]

MichaelG.

P.S. It's also worth looking here

P.P.S. I just found this excellent paper about Thermocouples
 
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ed1066

Member
@ed1066 - It will be hard to get the resolution you hope for whatever sense unit you are using since there are many mechanical and time factors involved - slightly different mounting, lead wire lengths response times and thermal coupling etc... I suppose you are looking for "steady state constant flow" sorts of BTU gain and such. .
Yes, you are spot on here. I seek to display the currently collected Watts and, especially, total kWh collected since commissioning. The idea is for users to see if they are getting the most from their system or are undermining it by switching on the gas boiler at the wrong time etc. So the heat gain observation is not absolutely critical to the functioning. However, I am careful to match thermal coupling etc. I also calibrate the DS18B20 sensors by comparing readings when water is circulated in controlled ambient conditions prior to installation, and store the matching factor in the 20X2 memory.

I've now 'made my bed and am lying in it' so to speak. I've gone for the DS18B20's and spent loads of money on the EMC, so I'm not able to change technology now.
 

premelec

Senior Member
@MichaelG - I was positing that constantan out and back would be hard to find - though I don't know what 'lead' wire for TCs is made of - I've made TCs with dissimilar TC wire... likely using TCs you'd want 4-20ma converter at a distance to avoid other troubles - TCs are great when high T in use!

@ed1066 - what are you using to measure heat transfer fluid flow accurately? And if it involves anti-freeze what's the specific heat of the fluid etc...? Good luck with your bed... :) Comparison readings should help confirming that the system is still operating well - one of the problems with custom systems is maintenance... My city has about 80 DHW systems in low income housing units, all of the same manufacture, and that makes it more likely for them to be maintained than the many custom systems made in the 1970s [also low income people report their electric bills going up when the solar quits and electric water heating elements kick in :) ] - many of which have been torn down for lack of maintenance [including 2 with about 100 collectors each with antifreeze deterioration]. So please try make whatever systems easily maintainable by tradesmen and women...
 
@MichaelG - I was positing that constantan out and back would be hard to find - though I don't know what 'lead' wire for TCs is made of - I've made TCs with dissimilar TC wire... likely using TCs you'd want 4-20ma converter at a distance to avoid other troubles - TCs are great when high T in use!
premelec,

We used Copper/Constantan twin-core leads [i.e. one core of each metal] sheathed in PTFE, and simply made the thermocouple by arc-welding the end whilst it was shrouded in an inert gas [N.B. this is essential].

Lengths were anything up to about 20 meters, and the test temperature range was commonly -30°C to +70°C
We calibrated every Thermocouple before use, and very rarely found a failure; generally they were within 0.1°C of the theoretical curve.

With clean welds, no extra junctions, and top class instrumentation; this arrangement is very reliable.
... We were an accredited Test House, testing to DEF STAN and Mil-Std specifications.

MichaelG.
 

premelec

Senior Member
@MichaelG - yes I have used that also - however it doesn't get rid of the copper tempco though it would halve it for out and back. I agree that's good way to go for a lot of applications.. The issue I was addressing was the variation of lead wire resistance to PT1000 or thermistor resistance sense units. It's gotten easier - though not cheaper - with the thermocouple ICs - no more mechanical chopper amps fake ice points and such... Sounds like you had a good job there!
 

ed1066

Member
@ed1066 - what are you using to measure heat transfer fluid flow accurately? And if it involves anti-freeze what's the specific heat of the fluid etc...? Good luck with your bed... :) Comparison readings should help confirming that the system is still operating well - one of the problems with custom systems is maintenance... So please try make whatever systems easily maintainable by tradesmen and women...
Thanks. During commissioning I put a flowmeter in the circulation loop and measure flow for various pwm settings. I then assume the relationship holds, with a correction for temperature. I don't want to have the flowmeter there permanently due to the expense and something else to go wrong. The only moving part in the whole system is the pump rotor.

The system is drainback so no antifreeze, hence a simple specific heat of the fluid value and also no related maintenance requirement. In fact it is maintenance free - if its working then there is nothing to do on maintenance throughout its life. It is possible it runs low on water after a couple of decades and then the OLED displays a message explaining a simple top up with tap water. As the system has not been in operation for a couple of decades my "maintenance free" claim is unproven, to put it mildly. But that is the design aim.

I will need to think of the various repair tasks - like pump replacement - and make it easy with good instructions.

Great to hear of the successes prior to the antifreeze problem.

By the way - how can a solar DHW system have 100 collectors?
 

premelec

Senior Member
Only one moving part is great! - I thought I'd mention a case where 7 drain back collectors split when a remote property had a power failure and there were 2 zone valves in the system - a plumber had assured the owner everything was great... However when the power failed in the daytime the two zone valves shut as they were NC types with auto shutoff ..... and the collectors froze over night!

100 collectors is not easy - big roofs lots of pumps big storage tanks - Ok perhaps I amplified the number... only dozens... commercial buildings. I think we still have a commercial bus garage with a whole lot of hot water collectors still in operation - and a recreation center with a mess of hot water units - one day I saw steam pouring out of their air bleeds as they were re-filling in daytime.

Stan [Manuka] has suggest a number of times that we'd do better with cheap PV panels and electric resistance tank heaters... [he has referred a paper not very recent where it worked out]. Anyhow I'm sure someday this will be the case as PV price and conversion efficiencies improve. That makes measurement and energy connections a lot simpler... And all you have to do is clean bird poop and dust... [that reminds me a friend was on a project in Africa where they had constant dust problems with PVs and sand storms - he didn't know why they didn't have wind power going... :-0 ]

Hope your system runs well forever trouble free... !
 

ed1066

Member
Stan [Manuka] has suggest a number of times that we'd do better with cheap PV panels and electric resistance tank heaters...
I'd say, if you have PV try all you can to use the electricity properly. In general electricity has about 2.5 times the carbon footprint of heat kWh for kWh. If you are away from the gas supply, maybe things are less clearcut.

PV is unlikely to get near the 70 to 80% efficiency of solar thermal, so it will need more roof space even if it costs less than solar thermal. I would then advocate considering the best use for the roof - all PV or mostly PV with a couple of panels for solar thermal. Minimising the panel area is important for many people as well for asthetic reasons, so they may prefer using solar thermal panels to heat water even if a PV system is cheaper. A final point is that in part-shading solar thermal is not undermined in the way PV is, so put solar thermal panels on any part-shaded patches.

My starting point was that solar thermal is too expensive for the benefits and my whole project has been working on getting the cost down. I think I can halve them. Now I'm at final verification prior to marketing.

premelec - you clearly know your stuff and have heaps more experience than me. Could I send you a pdf on the system for you to pick apart?
 

BeanieBots

Moderator
I'd like to add a few more complications to the thermal vs PV debate.
In the UK, it's quite common to get several 10 - 15 minute bursts (or even shorter) of sunshine during a day. This is particularly true at this time of year. With PV, you can instantly convert that into hot water with a controlled immersion heater. Would you be able to make any use of such short bursts using solar thermal after taking into account thermal lag?

The other question is, what is the minimum irradiation that each requires to produce anything useful?
With PV, anything over the grid-pull can be used.
How does that compare to what is required by solar thermal? (obviously, it's panel size dependant so not a simple answer)

I think these questions could only be answered by somebody with BOTH systems so that an exact like for like comparison can be made (theory is OK but requires a lot of assumptions about losses etc). Even then, I would suspect that there would be significant differences depending on what the starting temperature of the stored water is.
 

Jeremy Harris

Senior Member
I'm currently building a passive house, and faced the decision as to whether to fit a solar thermal system or just add more PV. It turned out to be a no-brainer to just add more PV, much, much cheaper (even though it's theoretically less efficient), always heats the water to some extent even when solar irradiation is small and the tank is already warm and best of all, no pipes going through my super insulated, airtight walls/roof.

A chap called Ed Davies has modelled the comparative performance/cost of using PV versus solar thermal for hot water and it turns out that much of the time there are substantial benefits in using PV, even though in theory the peak efficiency is only about 1/3rd of a solar thermal set up (in terms of collector area). Ed's write up on it is worth reading, here:http://www.edavies.me.uk/2012/01/pv-et-flat/ Particularly see the conclusion at the bottom of that page:

The big surprise, at least to me, is that below a bit under 400 W/m² the PV starts to be cheaper than the flat panel for 45 °C water and below 300 W/m² it even beats evacuated tubes for the same temperature difference. Down nearer 200 W/m² it's even getting into the same range as flat-plate for space heating temperatures (25 °C) though ET is still noticeably cheaper.

What this is saying is that if you want to heat some water to 45 °C in weak sunshine then using a PV panel and an immersion heater could actually be cheaper than using a solar thermal collector. This is contrary to what many might tell you - probably because thinking has not been updated as PV prices have dropped.
and more recently this post on his blog: http://edavies.me.uk/2012/11/pv-dhw/ which includes a handy calculator that allows one to work out just what the differences are between using PV to run and immersion heater for hot water and using a solar thermal set up.

PV is so very cheap now that if you have the roof area available then it seems a better, and potentially more reliable, approach. As we have 25 PV panels on the roof (a 6.25kWp array, south facing) it didn't make any sense, either financially or in terms of added system complexity, to look at fitting a solar thermal system for hot water.
 
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ed1066

Member
Thanks both to Jeremy Harris and BeanieBots. I want to look up the links and post a considered reply. But it will be in a few days now the Festive Whirl has engulfed me. Cherrio for now, Ed
 

premelec

Senior Member
@1066ed - I suggest you go to www,homepower,com [with dots..] and read their articles on troubleshooting hot water systems - this will give you a lot of information on what can go wrong.... and does. There is also a whole book, that I can't find the name of yet, on one installers 25 year experience if I recall correctly [I gave a copy to one of my kids...]. Perhaps these will clarify some issues for you. Nothing beats good passive design in the first place! [I'm a fan of Malcolm Wells who died not long ago and wrote several books on partially earthed houses]. Recently an architect claimed he didn't have to consider insulating shades and shutters as glass was so great now... it's much better and movable insulation is even better. I noticed a few years ago that removing my _exterior_ bug screens on south, east and west windows gave me 40% more solar energy through them... I just hadn't thought of it before...

Found the book on Amazon:

"Solar Hot Water Systems; Lessons Learned 1977 to Today" Spiral-bound – January 1, 2003 by Tom Lane
 
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ed1066

Member
In the UK, it's quite common to get several 10 - 15 minute bursts (or even shorter) of sunshine during a day. This is particularly true at this time of year. With PV, you can instantly convert that into hot water with a controlled immersion heater. Would you be able to make any use of such short bursts using solar thermal after taking into account thermal lag?
Thanks. I have a long post ready on your wider points. But on this I have a drainback system, so the hot water in the panel drains into a well insulated vessel when the sun goes behind a cloud. On start-up the metal in the panel has lower thermal capacity than the water so it re-heats quite quickly and then the pump comes on with the circulating water still hot.
 

ed1066

Member
Summary of this post

In response to Jeremy Harris and BeanieBots I here attempt to compare solar PV and solar thermal in the whole. The Ed Davies blogs that Jeremy links account for only the cost of the panel. The cost of the whole system is what matters. PV needs more area to get the same kW output and so the roof install costs are much more, meanwhile the internal plumbing for solar thermal is usually more expensive than the internal electrics for PV.

I take 3 looks at the cost-benefit of each option with 3 to 3.5 kW peak system:

(1) If I include the UK incentive system, then PV is better if a typical solar thermal cost is more than about £3,836 (spurious accuracy of course).
(2) Next if I ignore incentives and just consider the cheapest way to heat water, then PV is better if a typical solar thermal cost is more than about £5000
(3) Finally, if I just consider the best way to reduce carbon footprints, then PV is better if a typical solar thermal cost is more than about £2210

Actually a typical solar thermal cost system currently does cost about £5000. So PV is better, however my whole project is to reduce the costs and I am aiming for something like £2,700. So solar thermal is then better except on the carbon footprint measure.

But note that the carbon footprint assessment assumes that the precious PV electricity is not dumped as mere heat into a hot water tank. Please don’t do this, especially if the UK public is giving you a large incentive to generate electricity in order to reduce the UK’s carbon footprint. (But if you do, I blame the UK government for producing this perverse and pernicious incentive. )

The larger roof area required by PV in itself is not included, so a roof including both systems is possible. Also many may feel that a large PV panel is an eyesore, and may detract from the value of a house.

A final point is the news stories about Chinese manufacturers dumping PV panels in the European market. The low cost of PV may not be permanent and technology-related, but let’s hope that it is.

The rest of the post just fills out the details on the above.

Apologies to all you non-UK residents.

Comparison of financial return including UK incentives

I have compared the outcome from a Solar Thermal and a Solar PV installation, with each collecting about 3 to 3.5 kW at peak. The roof area was not a consideration in the calculations, but of course the PV will require about 4 to 5 times the roof area to collect the same kWh. This usually limits the kW peak, but incentive payments drop above 4kW.

I used an Excel spreadsheet with the “NPV” function – “Net Present Value” which adds up all the costs and benefits of a project allowing for the way that future benefits are less valuable now than equivalent immediate benefits.

For the solar thermal I used the most typical of the sites I have installed in my early applications: a family house with 2 flat panels and a predicted 2442kWh per year collection. The incentive is the Renewable Heat Incentive (RHI) and will be paid at 19.2 p per kWh for the first 7 years. Note that the RHI scheme is not quite yet full confirmed and set in stone by the UK government.

For PV I used the sites listed at the bottom of this post.

The calculation showed that the £5000 PV system generated sufficient income to give a positive NPV of £1,885 (i.e. invest £5000 to get back the £5000 plus £1,885 more).

For the solar thermal system to achieve the same NPV the amount invested had to be smaller as the high returns only lasted the 7 years of the RHI. The solar thermal gave the same £1,885 NPV if it cost £3,397. Any higher solar thermal cost and you are better off with Solar PV. You could argue that the NPV as a % on initial investment is the better measure. In this case the PV’s NPV is 38% of the £5000 investment. The same 38% return is achieved with solar thermal if it cost £3,836. So solar thermal must be cheaper than that to be the better investment.

I am aiming for something like a £2,700 cost, which is a 96% return. (Total return, not annual).

Comparison of costs to achieve water heating – no incentives

Now the comparison without UK public subsidies and incentive payments. The certification of my solar thermal installation gave a predicted annual generation of 2442kWh for the 3kW peak power system. The numbers below (£400 per annum at 14.9p per kW) suggest 2685kWh. However the solar thermal number includes the effect of not being able to use all the heat collected on long sunny days because the water use can’t keep up. A normal solar tank can be fully heated several times on a long summer day. The PV does not include this effect because all energy collected can be used or exported to the grid. If we assume the PV is used just to heat water, then the annual kWh collection will be much less. The cost of installation will be a little less as well because the electricity need not be converted to 240V and set to be in tune with the mains. On the other hand if it is not 240V the immersion heater will probably need changing which involves plumber costs.

The complications discussed in earlier posts – efficiency at below peak sunlight, effect of ambient temperature compared against water temperature, short bursts of sunshine and the thermal lag in a piped system, etc, are in theory accounted for in the sums above. However, I imagine both the PV and solar thermal calculations of annual kWh are quite ropey.

The conclusion of this is that equivalent peak kW systems will do a similar job in heating water, with the solar thermal doing a bit better. The cost of the thermal system should therefore be cheaper – so less than about £5000, to be better.

Comparison of carbon footprint – no incentives

Now the comparison from the point of view of reducing carbon footprint. Here there are 2 key assumptions (1) – THE PV ELECTRICITY IS NOT DUMPED INTO THE HOT WATER TANK – but is correctly used as electricity, this is vital and quite a bugbear of mine. And (2) the solar thermal system displaces water heating using gas burnt by a modern efficient boiler. If it displaces electricity, oil or LPG the solar thermal will become more attractive.

The results show that the solar thermal system must cost only £2,210 to match the £ per CO2 saving of the PV system.

PV Costs etc and Carbon Footprints Web search results

http://www.energysavingtrust.org.uk/Generating-energy/Choosing-a-renewable-technology/Solar-panels-PV#3

Updated January 2013

“The average domestic solar PV system is 3.5 to 4kWp and costs around £7,000 (including VAT at 5%), with the typical cost ranging from £5,500 to £9,500.

Costs have fallen significantly over the last year.”

http://www.theecoexperts.co.uk/how-much-do-solar-panels-cost-uk

“The average family home needs a solar PV panel that provides about 3kW of electricity. This will cost between £4,000 and £6,000 and cover about 21m² of roof space.”

This page also gives the latest info on annual returns including the feed in tariff: FiT £400, sell back rate £60 and savings £100.

http://www.theecoexperts.co.uk/35-kw-solar-panel-system

Published: Friday, 11 October 2013

“systems of the 3.5kW size will generally cost you around £4,000 to £6,000 to install.”

Carbon footprint numbers were from:

http://www.carbontrust.com/resources/guides/carbon-footprinting-and-reporting/conversion-factors

Download the pdf and it gives, page 4:

Electricity is 0.44548 kgCO2 e per kWh, natural gas is 0.18404 kgCO2 e per kWh. 100% electricity use efficiency assumed, but 85% for a modern gas boiler.
 
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