HVAC SYSTEMS - HFT Stuttgart

s m sc
s
h , col , i h , ad , i load , i loss , i cc , i fc , i
Page - 23 -
CHAPTER 02
dT
= Q&
+ Q&
− Q&
−Q&
+ Q&
+ Q&
(2.2.1-20)
dt
For the calculation of the energy exchange between the layers from top (i=1) to
bottom (i=n) first the effective mass flows are calculated. For two connections
(supply and return) of the heat source side and two connections (supply and
return) of the load side with typically counter flow arrangement (supply to the
heat source from the bottom and supply to the load from the top), the effective
mass flow between the layers is calculated from the difference between both
mass flows. For the first and the last layer the effective mass flow is zero.
In the developed model ideal stratification charge systems are considered for
both, the heat supply and the return flow of the load. Therefore, in the model
first the temperature profile of the storage tank of the previous time step is
evaluated and compared to the actual heat supply and load return temperature.
The heat supply is then fed into the highest possible node with a temperature
less or equal to the actual heat supply temperature. The load return is fed into
the highest possible node with a temperature equal or grater to the actual load
return temperature. In the model the mass flow control of the heat supply and
load return is controlled by load Boolean switch parameters
h
load
δ (heat supply) and δ (load return).
i
i
Once the two layers with direct mass flow are known the mass flows between
the layers are calculated. From the new mass flows and the inlet temperatures
of the two sources the new temperature of each layer is calculated.
m& = m&
⋅ δ − m&
⋅ δ for i = 2,N (2.2.1-21)
i
h
h
i
load
load
i
m& = 0
for i = 1 and i = N (2.2.1-22)
i
A positive effective mass flow with energy gain from layer i-1 to layer i is
considered by the parameter δi + = 1. For no or negative mass flow δi + = 0. A