DSP Operation

If the Aquaculture plant is build near marine salt works the DSP could be a good alternative to a conventional Solar Pond. We are going to considered two typical side-by-side salt works with 1000 m2 surface and 2m depth represented by L1 and L2 (Figure 5).

The first basin is constructed in order to act as a Solar Pond (Figure 5:1) and will work in a regime adjusted to aquaculture operation with an energy extraction of 30 kW. The only operation to be done is to create the gradient in L1 (Figure 5:2).

This is usually done with a diffuser that fills the pond from 1.5 m to 2 m depth. Here a salinity gradient is created being upper zone with 3% (seawater salinity) and the bottom with 25% of salt concentration. This operation takes about one week. At this time L2 basin is in a stand by mode.

If we only consider losses by salt diffusion L1 can work for a long time (about nine years) (Figure 5:3). After this the NCZ of L1 will be homogenized (Figure 5:4). Nevertheless there are other kinds of instabilities that accelerate this process. On the other hand we need to alternate the operation of L1 and L2 ponds with a periodicity of one year.

When L2 begins to be constructed as a Solar Pond (Figure 5:2a) L1 can aid to raise the initial temperature by energy extraction transfer. After about one week L2 is in conditions to work (Figure 5:2b). In this way L1 and L2 are connected.

Fig. 5 — Diagram with the two Solar Ponds, DSP concept, L1 and L2.

If L1 is not completed homogenized (Figure 5:4) the process must be accelerated. There are several processes to do this. Nevertheless, the simpler one is to stop the heat extraction. This will quickly raise the temperatures at the SZ and will destabilize the pond (Figure 6).

After this stage is reached, one must regain the initial concentration of 25% at the bottom. The remaining water must be evaporated from the pond until this concentration is reached. Basic calculations give the rate of evaporation to reach the original salt concentration of 25%. Results points that we need 51 days for a pond temperature of 40 °С, 29 days for 50 °С, and only 10 days for 80°C.

100

80

60

40

20

0

 

t [month]

Tsz Tamb Qext

 

Fig. 6- 1000 m2 DSP without heat extraction.

 

2. — Conclusions

We may conclude that for Aquaculture facilities these kind of solar ponds, are well designed and considerable less expensive than conventional ones. We must emphasize that this concept needs an annual periodicity of construction that is the periodicity of marine salt works recovering. In this way we have always a Pond that is working. The key of the concept of Dual Pond is to standardized maintenance operations with an annual periodicity.

The results here presented are a first approximation and more work must be done to deal the concept with assurance. Nevertheless in a first insight it seems to work giving the thermal energy needed for the application in mind.

References

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[2] — T. Radko, "A mechanism for layer formation in a double diffusive fluid", Journ. of Fluid Mechanics, 497, pp:365-380, 2003

[3] — S. Grossman, D. Lohse, "On geometry effects in Rayleigh-Benard convection", Journ. of Fluid Mechanics, 486, pp: 105-114, 2003

[4] — M. Ouni and A. Guizani, H. Lu and A. Belghith, "Simulation of the control of a salt gradient solar pond in the south of Tunisia", Solar Energy, Vol.75, pp:95-101, 2003

[5] — S. A. Enein, A. A. E. Sabaii, M. R.I Ramadan, and A. M.Khallaf, "Parametric study of a shallow solar pond under the batch mode of heat extraction", Journ. of Applied Energy, Vol.78, (2), pp:159-177, 2004

[6] — C. Angeli and E. Leonardi, “A one-dimensional numerical study of the salt diffusion in a salinity-gradient solar pond", Int. J. Heat and Mass Transfer, 47,pp:1-10, 2004

[7] — A. Joyce. Lagos Solares: contribuigao para o desenvolvimento de uma tecnologia. PhD thesis, U. N.L.1992

[8] — H. Weinberger, " The Physics of the solar pond", Solar Energy,8, 2, 1964

[9] — F. Zangrando /’Technical Note-Simple method for establish salt gradient solar ponds", Solar Energy, 24, pp:467-470, 1980.