System description. Solar collectors

Two solar glazed panels heat up the heat carrier fluid of the solar loop. The absorbing total surface of the two collectors is 4 m[10] [11]. They are facing south (azimuth angle 0°) and have 45° with the ground. Neither roof integration nor shadows are considered. Collector parameters are: optical coefficient B = 0.81, firs order thermal coefficient k1 = 3.61W/m2K and second order thermal coefficient k2 = 0.0045 W/m2K2 . Stagnation temperature is 215°C’.

2.2 Storage tank

A 300 liters tank is used to storage the solar energy (figure 1). At the bottom of the storage tank an integrated heat exchanger supplies solar energy recovered by the panels. This exchanger is installed inside a stratifier high as 90% of the tank. The tank has also an auxiliary electric heater allowed to run between 22.00 and 6.00 if the upper volume didn’t reached the set-up temperature2. The upper volume heated by the auxiliary heater is 100 liters. No stratification is possible in the volume above the heater when in operation [1].

Tap hot water is delivered at 50°C by a thermostatic mixing valve. This one add cold water to the hot water from the tank.

ixer

 

S teiage tank

 

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E iectric heater

 

C ontrol device

 

C oU w ater

 

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Fig. 1. Layout of a domestic hot water solar heaters (HWSH).

2.3 Control device

A classic control device is an electronic box able to take two decisions:

• To run or stop the pump of the solar loop depending on the temperatures registered in the collector and at the bottom of the tank;

The new control device is able to decide also the set-up temperature of the auxiliary heater depending on the needs and on the weather forecast [2, 3].

3. System model

Presented model is a finite difference one with nodal discretisation, and is based on the mass and energy balance. The simulations are carried on by dividing the tank in many isothermal elements or slices, with equal volumes. For each element we take into account the next heat transfers and energy interactions (figure 2):

• Heat lost to environment,

• Heat exchange with n-1 element,

• Heat exchange with n+1 element,

• Energy input from the electrical heater,

• Energy input from the solar exchanger,

• Energy input from the n+1 element when hot water is poured,

• Energy output to n-1 element when hot water is poured.

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