Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: jeannieboef@xxxxxxxxx
Date: Thu, 17 Nov 2011 16:55:23 +0000
Subject: [EnergyPlus_Support] water-water plate heat exchanger
I am trying to simulate a water-water plate heat exchanger. The district heat comes in on the source side, goes into the HX and out at a lower temperature.
This is possible because the source is heating the tank. When the tank is at the setpoint temperature as the source inlet temperature, there is no heat transfer.
If the source is another tank, I do not think that the second tank can be charged to the same temperature of the first tank. What you are doing is makeing the two tank temperature equal. This is one reason why the heat recovery at OA inlet does not work well. The precess is described in the Engineering Manual, I think.
On the user side, water comes in from a Low Temp Radiant system, enters the HX and leaves at a higher temp. The HX has a rated capacity of 66 kW.
The heat is being removed from the tank. The high temperature is the temperature of the tank water. As soon as the cold water comes in, the tank temperature will fall. 66kW is power. 66kw is needed from the district heating source to maintain the tank temperature. If the sourse is zero. The tank volume (mass) x specific heat is the energy stored.
I have modeled this using a waterheater:mixed object, setting the electric heating to zero so that it uses the source water only to bring the tank to the user side supply temperature of 32°C. Works just fine, but for the fact that setting the tank volume too small, kills it. But having a 1 m³ tank volume instead of a 0.001 m³ (which kills it), brings a possibility for the user side to draw energy, momentarily exceeding the 66 kW.
When you run the DHW example, the delta temperature turns on and off the charge side. As you turn on the use side water, the tank temperature falls. When the delta is at about 5 to 10°C (as set) the charge side times on and stop when the tank temprature is at set point again.
1) Can this simulation be done differently using waterconnections objects or a combination?
2) What is the minimum tank volume here, or does it depend on many factors such as flow rate etc.? The 66kw can be thought of as a source. Roughly, 8000 kW will raise 1 litre of water by 100°C in one sec.. ( 80 J = 1 cc 1°C x 80 s.h. of water)
3) Are there plans to support a water-water HX object?
What is the water to water HX object? It is a cooling tower of 66kw capacity, if you want this amount of heat to be conducted away.
A 60 gallon water tank with a 6 kw heater needs quite some time to heat up from 10°C to 60°C. It is turned off at that time. It it is left on for twice the time, the water will strts to boil. So how long is the 66kw heat will be on and to be absorbed.
Regards,
Jean Marais
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