modelResistiveWaterHeater

Information

Electric hot water tank with a single temperature node: the temperature is assumed to be homogeneous (no stratification).

Hypothesis and equations

The tank is supposed cylindrical: diameter d and height H.

The insulator (conductivity lambda and thickness e) is uniformly distributed on the outer surface of the tank.

The mass of water is supposed to be at homogeneous temperature T_tank.

The heat storage in the tank results from the superposition of three heat flows:

• the electric power injected into the water with a Pelec power resistance to maintain the setpoint T_sp according to an ON/OFF signal

- It is a hysteresis regulation with a half-band dT on both sides of the setpoint: Hyst = if T_sp+dT then 0 else pre(Hyst)

- The electric power injected into the water is equal to: Pelec = OnOff.P.Hyst

• the power participating in heating the drawing rate that enters in the tank with a temperature T_cold (cold water) and exits at temperature T_tank,

- The water heating power is equal to: debit.Cp.(T_tank - T_cold)

• tank losses through its envelope are calculated by reference [1]:

- An average coefficient of outside exchange is assumed: he = 10 W / (m².K)

- With a first approximation, the loss coefficient is equal to: KS = (1,1 +0,05/V).h.S avec 1/h = 1/he + e/lambda, S = pi.d.(H + d/2) et V = pi.d².H/4

Bibliography

[1] : E.C.S. : hot water in residential and tertiary buildings

Design and computation of facilities

Collection of AICVF guides, pyc edition, first edition 1991

Instructions for use

See example DHWResistiveWaterHeater.

Known limits / Use precautions

This model is very simple to implement in a study and enables a low computation time.

It is very sufficient and accurate to deal with consumption problems over a long period (1 year for example).

But for studies on power demands, it is advisable to use tank models taking into account the thermal stratification.

Validations

Validated model - Hassan Bouia 10/2012

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BuildSysPro version 3.6.0
Author : Hassan BOUIA, EDF (2012)
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