Structural Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 205

consumptions (such as the consumption of the recirculation pumps or the con-
sumption of the heat exchanger ventilators). Also, a third part might exist, namely
that which is dissipated on the outside, as it cannot be used. To exemplify, notice
that the output of the CHP unit consists in electric and thermal output, produced in a

fixed ratio c . If, at a certain point, the ratio between the electric demand and the
thermal demand is not equal with c , then the CHP unit produces energy in excess
under one of these forms. This excess could be stored or dissipated on the outside,
last case representing a loss. On the other hand, for each consumer, the input is the
total energy used to satisfy the residence needs, including the consumer

' s own

energetic needs, and is called consumption.
Between consumption and load there is a causal relationship according to which
the load must cover consumption. If every time this causal relationship is satis ed,

then the load diagram would be identical to the consumption diagram.
It is dif cult (and sometimes impossible) to control the system so that this causal

relationship to be constantly respected. Therefore, between suppliers and consumers
is interposed storage and the causal relationship is substituted with two others,
namely:

(a) permanent, the storage must cover the consumption, and
(b) periodically, the suppliers should fill storage, so that the amount of energy

contained in it to not exceed two limits considered acceptable.

Therefore, although the system controller continuously adjusts the load so that
these conditions are observed, however temporarily, the values of consumption and
load come to be different. This is because that in the short periods of time between
consumption and load there is not a direct and immediate relation. As a result, the
load diagram is different from the consumption diagram.
More speci cally, the system controller controls the load according to its control

law. This law establishes the momentary load depending on the amount of energy
that is available at that moment in storage. In turn, the control law is established
from the basic condition that must be followed, namely every time the cumulative
difference between consumption and load does not exceed the capacity

Eof

D

storage (see Fig. 26 a).
In order to get a more practical formulation of the basic condition let us analyze
the evolution of consumption and load during one day (see Fig. 26 b).
It can be seen that this condition is met if:

maximum value of any of the hatched area is E;

￿ D

two successive areas must have signs opposing values.

￿

At the same time we should note that the PQRS surface area is the daily con-
sumption and is equal to the P 1 Q 1 RS surface area representing the daily load and
also with the P 2 Q 2 RS surface area that represents both, the daily load and the daily
consumption. Also P 2 Q 2 line represents both average load and average consump-
tion. Note that the ratio of the maximum load and the average load could be
considered as a measure of the level of smoothing of the consumption diagram
when it is transformed into the load diagram.