[Digestion] Biogas for milk cooling centres in Central Uganda

David david at h4c.org
Wed Oct 9 15:28:52 CDT 2013



Art,

On 10/8/2013 12:27 PM, Art Krenzel wrote:
> Stephen:
>
> Take a look at this company who produces a modern commercial sized 
> thermal refrigerator system.  This unit may be too large for your 
> application but it is easier to downsize a design than upsize.
>
> https://bay169.mail.live.com/default.aspx?n=77550718&fid=1&mid=069dc4c6-3049-11e3-a4d7-00237de46188&fv=1#n=14993950&fid=2&mid=dec4048c-3030-11e3-b579-00237de3f534&fv=1 
>
>
> The Sevel sized refrigerators are quite small for most commercial 
> milk applications.

The link is specific to you, given that it requires a log-in...


More to the main point, we would have to ask Stephen Etheridge for his 
thought, but I took it that when he asked Samantha Carter "Why not use 
a gas fridge?", he was referring to absorption cycle refrigeration, 
the technology used in the Servel or Dometic refrigerators, of 
whatever size. As you may recall, strict or "pure" ACR uses only heat 
for power, whereas most modern chillers, AC systems and refrigerators 
use an additional or a different power source to provide for 
mechanical compression, which greatly increases what stands in the 
stead of ordinary efficiency for most such applications, which is the 
coefficient of performance.

As you no doubt realize, COP is used rather than ordinary efficiency, 
where COP is the ratio of the heating or cooling which results when 
divided by the amount of supplied energy consumed. In the case of heat 
pumps the ratio can be greater than one. (Indeed, a COP of 4 or 5 is 
not uncommon.) In other words, the energy provided by the ground or 
air to the heat pump system is not counted, such that one ends up with 
a system which can appear to be or be calculated as well better than 
100% efficient, an outcome disallowed by thermodynamics.

By contrast, the COP of a pure ACR system might be 0.15-0.25, or, 
where there is electricity available to pump the working fluid around 
(without compression), and blow air or pump water over its cooling 
coils, that COP might be increased. (Thus as compared with a system 
that uses a heat pump and has a COP of, say, 3, an ACR system with a 
COP of 0.25 is rather less than 10% as efficient.)

Of course, to start back at the beginning, where one has only biogas, 
and refrigeration is wanted, then pure ACR is the AD-powered option, 
even if it is inefficient. And where electricity /is/ available, then 
ACR makes limited sense from a COP standpoint, whatever the scale of 
the operation.


Samantha said, of course, that the present thought is to use what is 
already installed-- a unit, as I understand it, that requires 
electricity-- so in the specific case, the point is moot, but again, 
in the general case, where there is enough biogas to produce 
electricity, it is generally better to do so and use that energy to 
provide cooling.




d.
-- 
David William House
"The Complete Biogas Handbook" |www.completebiogas.com|
/Vahid Biogas/, an alternative energy consultancy |www.vahidbiogas.com

|
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"Make no search for water.   But find thirst,
And water from the very ground will burst."
(Rumi, a Persian mystic poet, quoted in /Delight of Hearts/, p. 77)

http://bahai.us/
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