RE: [EnergyPlus_Support] Heat Recovery Problems

["Ellis, Peter" <peter_ellis@xxxxxxxx>]

The "source rate" and the "use rate" refer to the heat gain rate (or loss rate) through the two sets of plant loop connections to the water heater.  In your case, it sounds like you have the "source side" connected to the IC engine for heat recovery.  I would guess that your "use side" is connected to some hot water load, like domestic hot water, or maybe space heating.  The heat gain rate (or loss rate) for either side is calculated as the Q = mass flow rate * Cp * (Ttank - Tinlet).  
 
That said, it is not often the case that the source rate and the use rate will match each other.  That is usually why the water heater has some volume to it for energy storage, and why there's a burner or electric heating element to provide auxiliary heat when necessary.  If your water heater has an auxiliary heat source, you need to take that into account too.  The report variable for that is "Water Heater Heating Rate".  You may also have parasitics adding extra heat to your water heater also.  
 
Finally, keep in mind that the heat rates will only add up if the water heater is kept at a steady-state temperature.  If, for example, you do not have a burner on your water heater and the use rate is greater than the source rate, the water in the tank will cool down until there's no more useable energy left.  Or conversely, if you are adding more heat through the source side than you are using, the tank temperature will heat up until it hits the Maximum Temperature Limit at which point the heat is vented away and lost.  
 
As for the IC engine heat recovery, you should be able to verify the actual heat recovery rate delivered by the plant loop by looking at the mass flow rate through the IC engine, the inlet water temperature and the outlet water temperature.  This calculation should match the water heater source rate.  There may be some reason why the "total heat recovery rate" of the IC engine is defined differently, I don't know.  Hopefully, they match.  
 
If this hasn't explained your problem yet, you can post the file.
 
 
-----Original Message-----
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]On Behalf Of energypluser
Sent: Sunday, September 03, 2006 3:19 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Heat Recovery Problems



Dear all

I connected a water heater to an IC engine for the purpose of recovering the exhausted heat.
When I was checking the results, I found something that I can not understand.
First, I found that the source rate of the water heater is much larger than the use rate. The 
difference is around 12kW in the winter design day and 7kW in the summer design day.
I checked that the heat loss rate is around 0.3kW in design days.
Why are some heat missing?
Second, I found that the total heat recovery rate of the IC engine was around 28kW in the 
design days. It is bigger than the source rate of the water heater! So the same problem 
appears again: there are some heat missing.
How can I explain this kind of things?
Thank you in advance!

mac.cz



 


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