[Stoves] Determination of thermal efficiency using the SeTAR Hetergeneous Test Protocol

[Crispin Pemberton-Pigott]

Dear Thermally Active Friends

 

Determining the thermal efficiency of a stove as a space heater or as a
cooker:

 

The SeTAR HPT Spreadsheet calculates the thermal efficiency of a stove used
as a space heater based on the elemental fuel composition (ultimate
analysis), the stack temperature and gas composition.

 

It uses the Siegert Method of calculating thermal efficiency. This is
standard in Germany and much of Europe. It considers the ‘chemical losses’
of CO not burning to CO2. It calculates using the HHV and considers that
heat released combustion of Hydrogen in the fuel has produced water that was
condensed and cooled to the ambient air (actually the stove air supply
temperature).

 

See Page 2/14 of 

http://www.docstoc.com/docs/70550236/Enthalpy-of-Formation-Worksheet 

 

This document shows both outputs values for some realistic cases. See the
difference between the ‘Standard’ efficiency as calculated in the USA and
the ‘Siegert’ efficiency. The cause of the difference is that the
calculation methods are quite different.

 

Normally the Siegert efficiency value is higher but you can see on pages 5/
and 9/14 that the Siegert calculated efficiency is lower than the Standard
Efficiency when the stack temperature is high.

 

In fact, the fuel heat number used in the US (in some industries) is not the
LHV value which assumes that water vapour and gases are cooled by the heat
exchanger to 100C and that heat was captured (the normal way to get LHV).1

 

Some industries use 150 C as the practical limit for extracting useable heat
so the ‘loss’ is an additional 50° C for everything passing through the
device. There are good (industrial) reasons for this, however there is
obviously confusion caused when trying to decide which method has been
applied by someone else.

 

The Siegert method is used in the HTP even if the instantaneous efficiency
is different some of the time. With space heating stoves, sometimes the HHV
is appropriate because the exhaust gases are condensing and going out as
white ‘fog’, sometimes even running down as liquid inside the chimney (the
black liquid that drops out) and sometimes it goes out as water vapour
indicating that the LHV is appropriate. So a decision was taken to use the
European Siegert method across the board for stating space heating
efficiency.

 

Prof Philip Lloyd (Cape Town) has reported difficulty determining the
thermal efficiency of ethanol stoves because of condensation of moisture
taking place on the bottom of pots (HHV) early in the test but later this is
not the case (LHV). However this situation being unusual, in general the LHV
is used for cooking. 

 

In the case of cooking pots on stoves, the same HTP spreadsheet uses the LHV
(in common with most protocols) because in the vicinity of the cooking
region of most stoves the likelihood and frequency of moisture condensing on
the pots throughout the cooking process is very low. If you were designing a
condensing heat exchanger instead of cooking, the method should be
appropriate to that case.

 

In summary: the SeTAR Heterogeneous Test Protocol uses the Siegert method
for determining the thermal efficiency as a space heater which means it uses
the HHV of the fuel, and uses the standard LHV values when determining the
efficiency of cooking over the fire.

 

These efficiencies are separately reported on the spreadsheet.

 

Regards

Crispin

 

1 [Also see S Baldwin’s discussion of this in his 1987 book – I agree with
his suggestion but it is not the standard method: it is that LHV values
over-report the actual cooking efficiency of stoves by .06 MJ/kg of biomass
fuels because of the enthalpy of gases between 100 C and the standard
ambient air source temperature.]

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