Kitchen Thoughts

Thirteenth lunar month of central heating Twelfth lunar month of central heating Eleventh lunar month of central heating Evaluation: a year from installation Tenth lunar month of central heating Index

Revised thoughts on efficiency and heat output

by Unknown - 15:43 on 31 October 2013

I have revised the table I gave for the first month to give what I believe to a better estimate of the Smart 120s output and efficiency. I have kept the old lower bound to make it easy to see the effect of my calculations, but I have included calculations for the residual heat as it dissipates heat on cooling and for the diversion of output because of the ESBE diversion valve in the system.

First lunar month of heating (revised)
Total kg pellets used	655kg
Average daily kg	23.4kg
Metered kilowatt hours to central heating/hot water	1726kwh
Gross kilowatt hour value of pellets	3144kg
Space heating lower bound	716.7kwh
Space heating including residual heat	1016.69kwh
Space heating including residual heat and ESBE distortion	1087.91kwh
Efficiency at lower bound	78%
Efficiency including residual heat	87%
Efficiency including residual heat and allowing for ESBE diversion	90%
Cost of pellets	£133.95
Potential RHI subsidy	£231.63

What led to the revision was Joe Fergusson's very useful feedback on my attempt to calculate the efficiency and heat output of the Smart 120. He said:

What raised my eyebrow was the thought of the cooker putting out 9kW into the kitchen. I know that when my 8kW log stove is working hard you can't stand near it for more than a minute. It strikes me that if the manufacturer says it can put out 5.2kW, then your assumptions leading to the 9kW figure might (I hope) be a bit out. Considering your estimated efficiency figures, my feeling is that a real world 80% for a boiler in this situation, i.e. where it is not working steadily a lot of the time, is good going.

Reflection on what he said has made me realise that my attempts to calculate efficiency and heat output were confused. However, now I have revised my thoughts they are actually more favourable to the Smart 120 than Joe's suggestion that it might be 80% efficient. The explanation of my revision is as follows.

Klover give figures for kilowatts to space heating and kilowatts to central heating/water for when the Smart 120 is at its maximum output (level 5) and its minimum output (level 1). They do not give such figures for the intermediate levels of output. I attempted to calculate them and then used the results for level 3 in my attempt to calculate our Smart 120's efficiency in the first month. For clarity, here is a table of the Smart's various levels of output.

Smart 120
Output Level	Space Heating (kw)	Central Heating/Hot water (kw)	Ratio of SH to CH	Source
5	5.2	17.5	0.3	Klover
4	5.09	14.25	.3575	interpolated
3	4.57	11	.415	interpolated
2	3.66	7.75	.4725	interpolated
1	2.4	4.5	.53	Klover

During the first month of central heating, we were running the Smart 120 at a maximum level of 3. I attempted to use the ratio in the table above for this level, together with my records for kilogrammes of pellets consumed and for metered kilowatt hours (to the central heating/hot water), to analyse the efficiency of the Smart. But my thinking was flawed. The figures for Space Heating and Central Heating/Hot Water in the above table are in kilowatts. Thus they are figures for the rate at which the Smart outputs heat to Space Heating and Central Heating (when it is at working temperature). However, to work out efficiency, what I need is a figure for the total amount of energy that the Klover outputs to Space Heating and Central Heating to compare the total energy value of the pellets it burns. The ratio in the above table is appropriate for calculating the direct Space Heating which the Smart provides while it is operating at full working temperature (this calculation is what resulted in the lower bound figure for efficiency in my earlier posting about the first month). But in addition to that heat output there is also energy which is stored in the Smart as it heats up and then released over some hours afterwards as it cools to room temperature. We can estimate this residual energy if we can estimate the average temperature of the Smart. I estimate that the average temperature of the Smart 120 at output level 3 is 160C (this is 40C below the top of the oven, 10C below the bottom of the oven and a long way below the centre of the boiling ring on the top which can reach 292C). Klover gives us a figure for the weight of the Smart 120 (which I think is a mixture of cast iron and steel). If I use the average of the specific heat for cast iron and steel in my calculation, then the residual heat which the Smart dissipates after is is switched off and cools form 160C to room temperature (20C) is about 5.34 kilowatt hours. During the first month of central heating we ran the Smart 120 from 6.30-10 in the morning and then from 16.30-22.30 in the evening, so I calculate that, in addition to its space heating output when working, the Smart 120 output a total of 10.68 kilowatt hours per day of residual heat while it was cooling (which seems very plausible to us as users).
In my posting on the first month, I did not work out the analysis in (2), but simply suggested that in our house the Smart 120 output a higher proportion of its heat to space heating than Klover's figures implied. One of my reasons for suggesting this was that there was residual heat (as described in (2)), but another was the distorting effect of the ESBE valve which is fitted in our system (see my earlier posting for an account of the ESBE valve). I still think that this will bias the system towards output to space heating, so I propose to allow a 10% increase in output to space heating and a corresponding decrease in output to water. This would mean that, at working temperature and output level 3, the Smart 120 in our kind of installation outputs at a rate of about 5.027 kilowatts to space heating and about 10.543 kilowatts to water.
I do not think my efficiency calculations should be accepted as definitive, but I do think they provide good reason to think that the Smart 120's efficiency in our house is well over 80% and is therefore tolerably close to Klover's claimed 90% efficiency.
As regards Joe Fergusson's criticism, which was the occasion of all this recalculation, the new proposal for the Klover's level 5 space heating in our house would be a much more modest rate of 5.72 kilowatts to space heating at working temperature. As regards the question of standing next to the Klover, even at output 4 which we have used for much of the second month I would not stand within inches of the furnace for as much as a minute. I have been told that, at output level 5, one might want to keep further away from the furnace - standing in front of the oven when cooking, but I have not tested this yet. However, as regards comparison with a wood burning stove, I think the Klover is a lot bigger than an 8kw wood burning room-heating stove, so it has a larger surface area over which to emit heat, and therefore even if it emitted 8kw I would not expect to emit such fierce heat to the front as Joe's stove
All this goes to show the benefit of putting up a public blog. Criticism enables one to improve one's thinking. I wonder what the next criticism will be?

Comment from Joe Fergusson at 10:43 on 01 November 2013.

Roger, your revised thoughts on efficiency and space heating behaviour seem logical and so very helpful. Thank you. My only remaining concern, vis-à-vis my plan for our smaller (I'm guessing) kitchen is that that our fridge-freezer sits directly opposite where the radiating furnace will be, less than three metres away and with nowhere else for it to go. Having said that, the appliance is so completely plastered with insulating children's art work, curling family snaps and postcards that for all the time the furnace is blazing the effect on the electricity bill probably won't be measurable! In view of your point 6, I should try to oblige with a criticism... I can only think to question the logic of allowing for the dissipation of the 10kWh of residual heat after switching off, without also allowing for the reduced projection of heat, to space or water, whilst that 10kWh is accumulating again in the body of the unit when it is getting back up to temperature again. I'd have thought these two would balance each other exactly, allowing them to be ignored and the steady state output used in your calculations. On another extremely minor matter, I note that you use a figure for the gross energy content of your pellets of 4.8kWh/kg. Can you tell us whether this figure comes directly from the supplier of these particular pellets, from a test certificate, or whether you drew this figure from some other source (and so perhaps a generic assumption)? I ask because this 4.8 figure relates to the calorific value of wood at 8% moisture content, a typical value for pellet, but if it is actually 7% or 9%, as it can be, this gives a range of 60Wh/kg more or less which, over a year, makes a difference of 1.25% either way - only enough to interest a real energy anorak like me! I should apologise for pedantry.... Joe F

Comments from Roger:
(1) Thanks for your comments.
(2) As regards the question of radiant heat, I think the Smart is designed to put a lot of heat into its cooking plate, as well as around the oven. So I think quite a lot of the radiant heat will go from its matt black cooking plate upwards. This is a good reason for having good insulation on the wall behind it, as we do. I suppose this heat ends up being convected around the kitchen, because our Smart heat it well and also heats much of the adjacent dining room. Our fridge and freezer are about 3.4 metres from the Smart, with a kitchen island in between. My concern about our fridge and freezer is with the air temperature in the kitchen rather than with radiant heat. Incidentally, it is not that the cupboard doors of the Island about 1.2 metres in front of the Smart become hot.
(3) I have put up a link to Stovesonline's page on the ESBE valve on the links page because it is relevant to my next comment.
(4) You are right that heat is stored in the Smart while it heats up and then released afterwards. If, while the Smart is heating up, water were being distributed as normal to the central heating/ hot water then my meter on the pipework would measure it, and we could use Klover's normal ratio(s) of space heating to central heating to calculate the amount of space heating from the meter's measurement. But the whole point of the ESBE valve is to enable the Smart to heat up quickly by preventing water from going to the central heating and instead routing it back to the Smart. Thus my meter will not record much heat whilst the Smart is heating up even though very substantial heat is being stored in the Smart. This is the basis of my treating the heat which is released when the Smart cools as an extra (I can see that the next step in this debate is for me to lurk in our cellar to observe how much the meter changes, if at all, while the Smart heats up!).
(5) The pellets we use our Balcas Brites. The figure I have for their value in kwh/kg is on the Get-Direct site which distributes their bagged pellets in Ireland. However we buy them in bulk in Scotland. I expect the actual value will vary.

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