MANTLE TANK DESIGN ANALYSIS

Calculations with MANTLSIM were carried out in order to investigate how the thermal performance of a small low flow SDHW system is influenced by the mantle tank design. The mantle tank design analysis is carried out with the mantle tank, Danlager 1000 marketed by Nilan A/S, as the standard reference tank. The design analysis is performed in such a way that only one parameter has been changed at a time in the calculation. Table 3 gives data for the standard reference system.

The circulation pump in the system is controlled by a differential thermostat, which measures the temperature difference between the outlet from the solar collector and the bottom of the mantle. The differential thermostat has start/stop set point at 10/2 K.

All the calculations in this chapter are carried out with weather data from the Danish Test Reference Year [11]. The daily hot water consumption is 0.100 m3 heated from 10°C to 50°C, which is tapped from the tank in three equally large parts at 7 am, 12 am and 7 pm. The yearly hot water consumption is 1674 kWh. The auxiliary energy supply system heats the top 0.082 m3 of the tank to 50.5°C and the indoor air temperature is 20°C.

The tank parameters that are investigated are the mantle inlet position, the mantle height, height/diameter-ratio of the tank, auxiliary volume and insulation of the tank.

Figs. 3-7 show calculated yearly net utilised solar energy of the system with the differently designed mantle tank. The standard reference system is marked in the figures.

Fig. 3 shows the calculated yearly net utilised solar energy of the system as a function of the mantle inlet position. The figure shows that the thermal performance of the system increases for the mantle inlet position moved down from the top of the mantle to a relative position of 0.35 from the mantle top, and that the thermal performance decreases if the inlet position is moved further down. The net utilised solar energy can be increased by 2.5% by moving the inlet port down to a relative position of 0.35. These results are in good agreement with the experimental results from the previous section.

Fig. 4 shows the net utilised solar energy as a function of the mantle height. The highest thermal performance is obtained with a mantle height of 0.25-0.30 m. The thermal performance can be increased by 5% by reducing the mantle from 0.43 m to 0.27 m. This is not in agreement with earlier theoretical investigations showing that the top of the mantle is best situated just below the level of the auxiliary volume, because this position maximises the heat exchange area without the auxiliary energy supply system heating the solar collector fluid in the mantle [8], [14]. If the top of the mantle is located above the level of the auxiliary energy supply system then the auxiliary energy supply system will heat up the mantle fluid and the thermal performance of the system will decrease.

The main reason for the new results is that the simulation model now takes the heat flow in the water in the inner tank into consideration. The heat flow in the water in the inner tank is caused by the upward fluid velocities along the tank wall during supply of heat from the collectors. Therefore the model calculates the thermal stratification which is built up in the hot water tank during periods with supply of heat from the collectors, not only in the mantle level of the tank, but also above the mantle. Another reason is that the mixing, occurring in
the mantle caused by differences between the temperature of the incoming solar collector fluid and the temperature of the solar collector fluid which is already in the mantle, now is taken in consideration by the simulation model. Therefore the simulation model now calculates the heat, which in periods with relatively low solar collector fluid inlet temperatures to the mantle is transferred downwards in the tank. This mixing will equalize temperature differences in the tank resulting in a decreased thermal performance of the system. With a reduced mantle height the influence of this mixing on the thermal performance of the system will be reduced.

The reasons for the increased thermal performance of the system by reducing the mantle height are a reduced tank heat loss due to the smaller mantle surface area and the increased insulation thickness, a decreased equalization of temperature differences in the tank in periods with relatively low solar collector fluid inlet temperatures to the mantle and the fact that the heat transfer area for the heat transfer from the solar collector fluid to the domestic water is not strongly decreased by reducing the mantle height. This is the case because heat by thermal conduction is transferred from the tank wall surrounded by the mantle to the tank wall above the mantle. Consequently, the heat transfer area used for transferring solar heat to the domestic water is a large part of the tank wall. However, the heat flow model used in MANTLSIM was developed based on CFD-calculations where the mantle covered either the lower half of the tank or all the tank height. The mantle height that gives the highest thermal performance in this study is when the mantle is covering less than one-third of the mantle height. There is a risk that when the mantle gets too small the model is not calculating the natural convection flow in the inner tank correctly and is over-predicting the effect of natural convection.

On the other hand, if these results are true, it opens a new perspective in the mantle tank design because less material can be used making the mantle tank cheaper, less heavy and therefore easier to install if only the bottom third of the tank should be covered by the mantle. There is therefore a need to verify by experiments that the small mantle height is able to create the degree of thermal stratification above the mantle calculated by the model.

Fig. 5 shows the net utilised solar energy as a function of the height/diameter ratio of the tank. It appears that the thermal performance increases with increasing height/diameter ratios. The net utilised solar energy increases by about 4% if the height/diameter ratio is increased from 2 to 5. When the height/diameter ratio is increased, the thermal stratification in the inner tank will also be increased, because the distance between top and bottom of the tank is increased and because the cross section area of the tank is decreased. The heat loss from the tank is also increased when the height/diameter ratio is

increased, but this is for height/diameter ratio between 2 and 5 overshadowed by advantage caused by the thermal stratification. If larger height/diameter ratios than 5 were investigated, the net utilised solar energy would at some point start to decrease because the increasing heat loss gets more and more dominating.

Fig. 6 shows the net utilised solar energy as a function of the auxiliary volume. It is seen that the thermal performance increases with decreasing auxiliary volumes. This result is expected as a larger auxiliary volume will increase both the auxiliary energy demand and the heat loss, and thus decrease the net utilised solar energy. The storage capacity for the solar energy is increased with decreasing auxiliary volume, and this also influences the result. The net utilised solar energy increases by 19% by decreasing the auxiliary volume from 82 l to 39 l. The auxiliary volume should be as small as possible while still meeting the demand of hot water.

Fig. 7 shows the net utilised solar energy a a of the insulation thickness of the sides of the tank. The figure shows that the thermal performance of the system increases with increasing insulation thickness at the sides.

A number of parameter variations have been carried out to reveal how the different tank parameters influence the thermal performance of low flow SDHW systems. In the following it will be elucidated how to improve the design of the Danlager 1000 mantle tank by relatively simple geometrical changes. The change of the design is made in such a way that one parameter is changed at a time in the calculations. Also here the data from Table 3 are used for the reference system in the calculations.

The following tank parameters are changed: height/diameter ratio of the tank, mantle height, insulation, thermal conductivity of the tank material and the wall thickness of tank and mantle.

Fig. 8 shows the net utilised solar energy and the solar fraction as a function of the different changes in the mantle tank design. The first column shows the thermal performance of the system with the Danlager 1000 heat storage with mantle inlet position at the top of the mantle.

The first change is the height/diameter ratio of the tank, which is changed from 2 to 4. The total volume and the ratio between the auxiliary volume and the total volume are kept constant. The second change is the mantle height, which is decreased from 0.72 m to 0.55 m. The third change is the insulation of the tank. It is assumed that the tank should fit into a cabinet with dimensions 0.6×0.6×2.0 m3, and by increasing the height/diameter ratio the outer diameter is reduced, and the side insulation is increased by further 0.05 m. The fourth change is the tank material, which is changed from normal steel to stainless steel. The fifth, and last, change is the wall thickness of tank and mantle, which is reduced from 3 mm to 2 mm.

The change concerning the insulation gives the most significant improvement, while the change of the wall thickness gives the smallest improvement. The net utilised solar energy is increased from 802 kWh/year to 1009 kWh/year by applying the mentioned changes in the design. It is an improvement of 26% of the net utilised solar energy.

2. CONCLUSIONS

An improved simulation model for low flow SDHW systems has been developed and validated by means of experiments.

Calculations with the model have shown that marketed mantle tanks can be strongly improved by relatively simple design changes: By increasing the height/diameter-ratio, by reducing the mantle height, by increasing the insulation thickness on the sides of the tank and by using stainless steel instead of steel as tank material.

The thermal advantage foreseen by the calculations by decreasing the mantle height is of great interest, since the cost of the mantle tank can be decreased by reducing the mantle height. However, it must be mentioned that the model is not validated for small mantle heights. It is therefore recommended to start investigations with the aim to elucidate if the model simulates the thermal behaviour of mantle tanks with small mantle heights in the right way.

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