Some thoughts on solar ponds

Three types of no-salt solar ponds, which do not exhibit environmental or operational problems of the kind observed for salt ponds, are examined on a laboratory scale. The negative temperature gradient necessary for heat storage was obtained by using appropriate arrangements in such a way that narrow passages appear which reduce substantially the heated water currents, or by using two (or more) transparent immiscible liquids which constitute a density gradient serving the same purpose. The influence of the dissolved air in the water mass is discussed.     

A parametric study on solar ponds

A linear relation between the efficiency of solar ponds and factor $\text{θ}{}_{\mathrm{d}}\text{H}$ which is the temperature difference between the pond bottom and the ambient divided by the average insolation is presented. This relation, which has been developed based on a steady state analysis provides valuable information on the relative importance of the parameters involved in the operation of solar ponds. It is found that the existence and the thickness of the top convective zone has a profound negative effect on the yield of solar ponds. The optimum thickness of the density gradient layer under various conditions is also presented. The effect of ground losses is discussed, and it is shown that for the case of wet soil, especially if the level of the underground water is high, the pond should be thermally insulated. It is also shown that the steady state analysis can predict with good accuracy the yearly average response of solar ponds under transient conditions.     

Membrane stratified solar ponds

The membrane stratified solar pond is a body of liquid utilizing closely spaced transparent membranes to quench convective heat transfer in the top part of the pond. Membranes may be configured as horizontal sheets, vertical sheets or vertical tubes. Several suitable liquids and membrane materials are discussed. Conditions for suppression of convection are described, and transmission of solar radiation through the pond is discussed for each of the three membrane configurations. The steady state thermal efficiency is calculated for the horizontal sheet configuration. Thermal behavior is similar to that of salt gradient solar ponds, but much deeper heat storage layers are feasible. In some cases aquaculture farming may be suitable in the storage layer.     

Wind-mixing experiments for solar ponds

Experiments are described in which wind-induced entrainment was measured in a laboratory tank under strongly stratified double-diffusive conditions in order to calibrate a numerical model for application to solar ponds. Results showed that there was no effect of double-diffusive stratification and that the entrainment followed approximately an inverse Richardson number relationship. Shear-induced mixing was found to play a strong role in the entrainment process, and it is suggested that an effort be made to obtain more data on wind-induced currents in operating solar ponds. An additional series of tests is also reported in which a preliminary investigation of the effectiveness of floating wave suppressors was carried out. These tests showed that the wave suppressors, consisting of either plastic netting or PVC pipes, were able to reduce average turbulence levels in the water, though it appears that there may be some enhanced mixing directly beneath the nets or pipes.     

Radiation transmission measurements for solar ponds

Transmission spectra and extinction coefficients for different salt solutions have been measured. The effect of pond transparency on the performance of a district solar pond is also discussed.     

Absorption of solar radiation in ponds

Analysis is presented to predict the local rate of solar energy absorption in a pond using the radiative transfer theory. The physical model considers absorption and scattering by the water and internal reflection of radiation from the air-water interface as well as the bottom. A forward scattering approximation and a discrete-coordinate approximation of the radiative transfer equation are discussed. Numerical results for the local volumetric rate of solar energy absorption in the water are presented in the paper for a range of parameters of physical interest. The effects of the directional distribution of solar radiation incident on the water surface, the attenuation of solar radiation by the atmosphere during the diurnal cycle and the modification of the spectral radiation characteristics of water by impurities and additives on the absorption and distribution of the absorbed energy in the pond are investigated.     

Solar radiation absorption in solar ponds

The local rate of absorption of the solar radiation in a solar pond is determined for the direct component at angles of incidence from 0° to 75° with 15° intervals as well as for the diffuse component by the exact treatment of the radiation problem. The effects of bottom reflection, the pond depth, the type of radiation on the thermal performance of the pond are examined, and a new rigorous approach is presented for treating diffuse radiation as a direct beam. The fraction of the solar radiation absorbed within the first 10 cm of water is determined under various conditions. The local rate of solar energy absorption at any depth and at any incidence angle can readily be computed from a fourth-degree polynomial expression, the coefficients of which are tabulated for different incidence angles and bottom reflectivities.     

Salinity profiles in steady-state solar ponds

The slope of the walls in a non-convective rising solar pond is shown to affect its advective-diffusive salt flux, and hence the steady-state salinity profile. The effect considered, unlike boundary effects, is independent of the pond's horizontal size.     

The influence of non-constant diffusivities on solar ponds stability

A solar pond is a basin of water where solar energy is trapped due to an artificially created gradient of salinity that prevents convective motions. The present study intends to clarify the contribution of non-constant diffusion coefficients for the stability of the gradient layer together with the influence of solar radiation absorption, the thermal and molecular diffusivities being assumed to be linear functions of the vertical co-ordinate z. The analysis shows that the consideration of these two effects decreases the margin of stability in comparison with previous studies based on a layer of fluid heated from below with constant diffusivities coefficients and linear profiles for both temperature and salt.     

Large scale solar cooling design using salt gradient solar ponds

The future of large scale cooling is closely linked to the long term economically viable component development for collection and storage of solar energy at relatively high temperatures. As such, solar ponds at the present state of their development are undoubtedly considered as the only promising large scale solar collection and heat storage device for such applications. The present analysis, based on numerical calculations, allows a parametric investigation of solar pond design and operational characteristics to the capacity of a conventionally designed, commercially available, absorption chiller. The results can prove very useful for the rough design and pond size selection for operation of chillers of a substantial capacity, directly from solar ponds.     

A study on the transient behaviour of solar ponds

In this paper, the transient behaviour of solar ponds has been investigated using a finite difference formulation. The performance of solar ponds can be successfully analysed and the effects of various parameters studied. The thickness of the layer with density gradient has a profound effect on the performance of a solar pond and an increase of the thickness of this layer from 1 to 2m increases substantially the operating temperature for the same overall efficiency.

The effect of the pattern of heat extraction is discussed. Heat extraction at a constant rate will result in large temperature fluctuations from summer to winter. But, if the heat is extracted at a varying rate proportional to the monthly average solar radiation, then the temperature fluctuation is considerably decreased as compared to the case of constant heat removal for the same yearly efficiency. It is possible to operate a solar pond in the Melbourne area with a yearly efficiency of 15% having a high temperature of 67 °C in summer and a low of 40 °C in winter.

The performances of solar ponds in Alice Springs and Darwin have been studied. It is found that the maximum bottom temperature does not go beyond 80 °C for a pond efficiency of 20%. Since these two cities have the most favourable locations for solar ponds, the indicated maximum temperature casts doubt on the prevalent view that a solar pond can operate at a temperature above 90 °C with an efficiency of 20%.     

Brine clarity maintenance in salinity-gradient solar ponds

Brine transparency is an important part of the maintenance of a salinity-gradient solar pond as it affects the amount of solar radiation reaching the storage zone and hence has an influence on the thermal performance. There is a wide range of factors that can hinder the transmission of light in a solar pond. Algal and microbial growths are the most common problems encountered in working solar ponds and control of their densities is essential to maintain transparency. Two different chemical treatment methods for algae growth prevention are described in this paper: chlorine and a novel chemical product – copper ethylamine complex. The latter method has never been implemented previously in a working pond. This paper discusses the theory of the algae control methods used and presents the experimental results of the chemical treatments. The results showed that Cupricide is more effective than chlorine and is therefore the recommended chemical for algae control in solar ponds; it improves the water transparency especially in the upper convective zone and lower convective zone with all measurement values less than 1 NTU. Chlorine was found to be more corrosive than Cupricide due to the acidic effect it has on the pH. The preliminary cost analysis showed that granular chlorine is the cheapest chemical. A more detailed financial analysis is nevertheless required to refine these costs.     

On the hydrodynamics of salt-gradient solar ponds

The objective of this paper is to discuss those hydrodynamical issues that affect the performance of the solar pond as an energy collector and storage system, e.g., mass and energy balance; formation, stability, and maintenance of the gradient layer; energy extraction from the bottom mixed layer; stability of stratified fluids to shearing flows; interface dynamics; and wall effects. Many of these topics are not fully understood and the discussion focuses on the present state of knowledge, some of the engineering correlations available at this time, and the research that is still required to resolve the relevant issues.     

Net energy analysis of residential solar ponds

A net energy analysis (NEA) of three different residential solar pond scenarios is performed. A single home, a complex of twenty homes and a community system with district heating are considered. The designs considered, Rabl-Nielsen and Krass-LaViale, are studied for locations in Columbus, Ohio and in Northampton, Massachusetts. The analysis reveals that economies of scale and design considerations influence the net energy ratio (NER).     

A parametric design study of solar ponds

The salt stratified solar pond is found to be a reliable solar collector and storage system. This paper discusses the effect of varying certain design parameters on pond steady-state temperatures. These significant parameters are sizing parameters—pond surface area and depth of the pond; operating parameters—storage volume and the heat extraction fraction; and geo-climatic parameters3s?olar radiation, water table depth and upper convective zone thickness. Studies indicate that there is an optimum depth and storage volume of the pond for each application in terms of temperature and heat load desired.     

A wind-mixed layer model for solar ponds

A computer model is described which may be used for predicting transient salinity and temperature profiles in a salt gradient solar pond. It is intended for use in modeling large surface area ponds where wind-mixing would be expected to play an important part in the dynamics of the upper layer. The formulation predicts the depth of the upper convecting zone using a mixed-layer model which incorporates the wind-mixing algorithm described by Bloss and Harleman [1,2]. This is in contrast to earlier solar pond models which have generally assumed a constant value for this layer depth. Results have been obtained for a number of 1-yr simulations of a large hypothetical pond in Richmond, Virginia, and these have been used in testing the sensitivity of the model to several of the input parameters, including the radiation term and the form of the wind-mixing algorithm. The model output is also compared with field data from an operating solar pond and good agreement is found. Results have indicated that some measures will have to be taken to counteract the mixing action due to wind stress, if the upper mixed layer depth is to be maintained at an acceptable level. The model is expected to be useful in large-scale pond design.     

Benefit cost analysis of gel solar ponds

A benefit to cost (B/C) analysis was performed on gel ponds based on experimental data collected from the circular demonstration gel pond (5 m dia × 1.25 m depth) located at UNM. The measured transmissivity, predicted temperature profile, and calculated surface heat losses with volumetric heat generation were used as physical data in the coded design model (GPDM) used for sizing the solar pond. These data were used in the coded economical analysis model (GPEA) to calculate capital, operation, life cycle, and cost of delivered energy for a specific pond. A general case study was considered to demonstrate the potential and economical feasibility of gel ponds as a source of hot water (45°C) for domestic use in five regions in the United States. An optimized gel pond showed B/C values as high as 1.35 for high insolation areas (Southwest, Puerto Rico) and as low as 0.45 for low insolation areas (Great Lakes, Atlantic NE) compared to 0.96 and 0.48 for salt gradient ponds of the same size and location. In a special case study to demonstrate industrial applicability of gel ponds as a source of hot water (65°C) for a textile mill (Cairo, Egypt), the optimized pond had a B/C value of 1.07 compared to 0.93 for an optimized salt gradient pond of the same load output. In general, for the same size (400 m² × 4 m deep), location (southwest) and extraction temperature (45°C), in gel and salt gradient ponds, the gel has higher capital cost (19%), lower operating cost (53%), lower delivered energy cost (26.4%), and higher extraction efficiency (32.5%). While, for the same load output (150 kW thermal) and location (Cairo), a gel pond has higher capital cost (21.5%), lower operating cost (63.5%), smaller surface area (21%), shallower depth (28.6%), and lower delivered energy costs (13%). Using GPDM (Gel Pond Design Model) and GPEA (Gel Pond Economic Analysis) computer programs, a sufficient engineering and economic analysis can be performed for gel and salt gradient ponds, respectively.