THE SEAWATER GREENHOUSE AND THE WATERMAKER CONDENSER. Philip Davies* and Charlie Paton

Transcription

1 THE SEAWATER GREENHOUSE AND THE WATERMAKER CONDENSER Philip Davies* and Charlie Paton * School of Engineering, University of Warwick, Coventry CV4 7AL, U.K. philip.davies@warwick.ac.uk Seawater Greenhouse Ltd, 2A Greenwood Road, London E8 1AB, U.K. charlie@seawatergreenhouse.com ABSTRACT The Seawater Greenhouse uses solar energy and the humidity of the ambient air to provide desalination and cooling. Its purpose is to provide a sustainable means of cultivating high quality crops, year round, in arid coastal regions. In this paper we review the concept of the Seawater Greenhouse mentioning the prototypes constructed in Tenerife, the United Arab Emirates and Oman. A key component of the Seawater Greenhouse is the condenser where fresh water is produced. The condenser exchanges heat between warm humid air and cool seawater. We review some theories of the condensation process and describe our experiments using two types of condenser: (i) a standard tube-and-fin condenser made of metal and (ii) the Watermaker condenser (developed specifically for this project) consisting of an array of polythene tubes. agricultural. Worldwide, agriculture accounts for 70% of all usage, and in arid regions the figure increases to around 90%. While the amount of rainfall (and therefore the renewable supply of freshwater) is roughly fixed, the world population is expanding. This suggests that water for agriculture will become increasingly scarce due to competition with industrial and domestic use. At the same time, agricultural output will need to increase in order to feed more people. In many parts of the world, desalination is the only method of meeting the shortfall. Yet desalination is expensive in terms of energy. Even with the more efficient desalination plant, 1 kg of oil can only yield about 1000 kg of freshwater. Using standard methods of irrigation, this amount of water may only yield about 1 or 2 kg of edible crop, see Figure 1. In conclusion, we review the current status of the condenser and future directions. INTRODUCTION An estimated 1.75 billion people live in areas affected by severe water stress (Vörösmarty et al. 2000). The many purposes for which humans require fresh water can be broadly categorised as domestic, industrial and 1 kg oil 1000 kg water 1 2 kg crop Figure 1: Basic economy of desalination for agricultural 1

2 From this simple discussion it is apparent that there are fundamental conflicts among the availabilities of energy, water and food. Population growth will make these conflicts increasingly acute. The Seawater Greenhouse responds to these issues at two levels. Firstly, it aims to desalinate seawater with the use of renewable (principally solar) energy. Secondly, it employs the resulting freshwater in a very efficient way in order to produce food. A key element of the Seawater Greenhouse is the condenser where freshwater is produced. This device is required to transfer a large amount of heat with a small temperature difference. It can therefore present a bottleneck in the process and we wish to report on recent progress in developing a new type of condenser referred to as the Watermaker condenser. THERMAL CYCLE OF THE SEAWATER GREENHOUSE The initial experiments with the concept were carried out in Tenerife in These served to demonstrate the viability of a condensation process in which the cold source is provided by the deep ocean. The approach is applicable at locations having a steep sea ledge allowing the water below the thermocline to be easily accessed, or where upwelling of cold water naturally occurs. A particularly interesting feature of the Tenerife Seawater Greenhouse was that it was ventilated by the wind alone, without the need for any fans. Following this project, it became apparent that there was a demand for a solution that avoided the need for the any special ocean conditions and the associated engineering. This led us to devise a self-contained thermal cycle, first implemented in the United Arab Emirates, see Figure 2, and more recently in Oman. The principle of operation of the Seawater Greenhouse is explained in Figure 3. On the left, air enters through a porous evaporator that is continuously moistened with seawater. The humidity of the air is increased to near 100%, causing the temperature of the air to drop. Inside the greenhouse, the crops are partially protected from the sunlight by the solar water heater, which consists of an array of pipes carrying seawater. Figure 2: The Seawater Greenhouse in the United Arab Emirates. 2

3 These conditions of increased humidity, plus reduced sunlight and windspeed, result in the rate of water loss through transpiration being reduced several-fold compared to cultivation of the same crop under ambient conditions. Having passed through the growing area, the air is heated by the sun and absorbs further water vapour transpired from the plants. As it passes through the second evaporator, its humidity is again brought to near 100%. The temperature of the air is increased by the fact that the second evaporator is fed with water that is heated by the solar water heater. At this point the air is hotter than the water leaving the first evaporator. The two fluids are fed into the condenser. Heat is exchanged from the air to the water and as a consequence condensation of freshwater occurs at this point. DESIGN OF THE CONDENSER The condenser has tended to constitute a significant cost of the greenhouse system, representing up to 20% of the total cost of approximately 30k to 40k for a 1000 m 2 cultivated area. This was the motivation for the development of the Watermaker condenser. The component parts of this new design are a fraction the manufactured cost of the tube-and-fin condenser. These components are not factory assembled but assembled on site. This means that it is economical in situations where low-cost labour is readily available, as is the case in many of the countries where the concept is of interest. Aside from the motivation of cost, it has been found that the new type of condenser also has some advantages in terms of performance. This has prompted us to investigate the underlying theory of condensation, with a view to understanding the process more fully and learning how to optimise it. Three theoretical approaches are summarised under the subheadings below. Standard heat exchanger theory The condenser is a type of heat exchanger in Solar radiation First evaporator Solar water heater Condenser Air in Attenuated radiation Air out Crops Air out Seawater top-up Growing area Second evaporator Freshwater out Figure 3: Principle of operation of the Seawater Greenhouse. Arrows indicate the flow of seawater. 3

4 which there are two phases (vapour and liquid) present on the airside. If we choose to analyse the condenser using the method normally applied to heat exchangers, this will allow us to access a large amount of results existing in that field. These are typically summarised by correlations of the form: Nu = f(re, Pr) [1] where Nu is the Nusselt number, Re is the Reynolds number and Pr is the Prandtl number. The dimensionless form enables us to apply the correlation to a wide range of physical scales and types of fluid. The standard theory of counter-flow and parallelflow heat exchangers allows us to formulate the temperature difference across the whole heat exchanger in the appropriate way using the concept of logarithmic mean temperature difference. Having applied such a correlation to calculate the amount of heat transferred, we then need to convert this into the amount of mass transferred since we are mainly interested in the amount of condensed water produced. A convenient assumption here will be that the process follows a saturation curve, with virtually saturated air (100% relative humidity) entering and leaving. This then ties the rate of condensation to the rate of heat transfer. The effect of the condensation process on heat transfer can be included by some empirical correction factor. Alternatively, we might assume that the whole process is governed by the rate of heat transfer through the air (sensible heat) with the latent heat and mass transfer following automatically. Aerosol formation Theory Although the above approach is easy to apply, it does not relate the overall process to fundamental physical principles. A possibly more rigorous approach to modelling condensation from air is presented by authors such as Clement (1985), or Barrett and Baldwin (2000), and others cited by them. This literature appears to stem from concerns surrounding the safety of pressurised-water nuclear reactors. A failure of such a reactor could theoretically result in water vapour being released into the atmosphere. Under certain conditions, the water vapour can form an aerosol capable of carrying nuclear contaminants over large distances. Although the design of devices such as heat exchangers and condensers may not have been what motivated this line of investigation originally, it has led to the modelling of systems that are quite relevant. Thus, Barrett and Baldwin (2000) consider the case of condensation from a gas carrying a vapour through a cool tube. The basic approach is to formulate coupled convection-diffusion equations for both mass and heat transfer, with boundary conditions defined at the tube walls. The solution becomes relatively complex and does not necessarily lead to a simple engineering correlation comparable to equation [1] above. Dew formation theory The above model tends to consider surfaces as simple geometrical boundaries. This is in contrast to the theory of Beysens (1985), whose motivation arises from studying the formation of dew. The theory developed by Beysens considers the energy of surface tension needed to cause a droplet to grow on a surface. In this approach it is therefore fundamental to consider the contact angle of the droplet meniscus against the surface. A low contact angle, corresponding to hydrophilic surface, will result in relatively easy formation of droplets that will readily merge into a continuous film. A high contact angle, corresponding to a hydrophobic surface, will make it more difficult for condensation to form, as it is necessary to continually overcome surface tension. We conclude that a proper engineering design method for condensers is, to our knowledge, lacking. The ideal formulation would perhaps be one that carries through the scientific rigour of the second and third approaches above, into a set of broadly applicable correlations comparable to those currently used in the standard heat exchanger theory. 4

5 EXPERIMENTS To assess the performance of the Watermaker condenser in comparison to the baseline provided by the metal tube-and-fin type condenser, the following experiments were carried out. The condenser was built into a duct such that the air flowed first through an evaporator and then through the condenser. The evaporator was fed with warm water, thus ensuring that the air entering the condenser was saturated in water vapour. The condenser was continuously fed with cold water. Four temperatures were monitored: air inlet, air outlet, water inlet and water outlet. In separate experiments, the condenser was switched between the two types. They both presented the same surface area of 1.25 m 2 to the air stream. The tube-and-fin condenser consisted of an array of 76 vertical aluminium fins, each measuring 160 mm high by 60 mm in the direction of air flow, at a spacing of 2.24 mm giving a total width of 170 mm for the condenser. There were 8 copper tubes each 15 mm diameter passing through the fins. The Watermaker condenser consisted of a 5 5 square array of vertical tubes, each 32 mm in diameter and 500 mm high, on a pitch of 38 mm between centres. The tubes were of clear polythene 70 m thick. Airflow was in a direction perpendicular to the axes of the tubes. Table 1 below gives some example results representative of our general findings. It can be seen that the temperature of the air entering the condenser varied among the experiments. Warmer air contains more water vapour and therefore we would expect it to result in more condensate being formed. The results have therefore been corrected (in the 8 th column of the table) for air entering at 25 ºC and leaving at 20 ºC. The mass transfer coefficient is based on the corrected rate of condensation (mg/s) divided by the log mean temperature difference and the surface area of the heat exchanger on the airside. DISCUSSION Table 1 indicates an increase in the coefficient of mass transfer, of about 50%, when the Watermaker condenser is compared to the tube-and-fin condenser. At first sight, this may be surprising as heat and mass transfer are understood to be diffusion limited processes governed by a characteristic length, with a shorter length resulting in more rapid transfer. This length should be related to the gaps through which the air flows. The tubefin-condenser has a channel gap of only about 2 mm, whereas in the Watermaker condenser the gap between adjacent tubes is 6 mm at the narrowest point. Table 1: Results of condenser experiments Test Type of condenser Fluid temperatures C Condensate flow ml/hour Coefficient of mass Air Water observed corrected transfer in out in out (mg/s)/k m 2 1 Tube-and-fin Tube-and-fin Watermaker Watermaker Watermaker

6 A possible explanation could be the regime of flow in the two cases. In the tube-and-fin case the Reynolds number was approximately 150 indicating laminar flow. In the Watermaker condenser, the Reynolds number (based on tube diameter) was around 3000 indicating a transition between laminar and turbulent flow. This could result in enhanced mixing and mass transfer. It was decided to compare the results of the experiments with heat exchanger theory provided by Isachenko et al. (1987, Chapter 9). This book draws on research results from the Russian literature. The correlations given for banks of tubes are as follows. On the airside, we have: Nu = c Re n Pr 0.33 [2] where c and n depend on whether the tubes are inline or staggered and depends on the pitch of the tubes relative to their diameter. While for the resistance on the water side, i.e. inside the tubes, we use: Nu = 1.55 (Pr Re d/l) 0.33 [3] Application of this theory to the experimental results led to the following preliminary conclusions: The theory is in reasonable agreement with experiment, provided that equation [2] is used to model sensible heat transfer, which is then assumed to govern overall heat transfer on the airside. The resistances to heat transfer on the air and water sides are roughly equal. The resistance in the thickness of the polythene material is negligible. staggered tubes, on a triangular pattern, are expected to give an improvement of about 16% to the airside heat transfer relative to in-line tubes on a square pattern. These findings have now been applied to the practical design of the Watermaker condenser. Thus a staggered arrangement of tubes was preferred over an inline arrangement. We have allowed an increase in the thickness of the polythene, to give extra robustness without significant loss of heat transfer. We have chosen a counterflow arrangement over a crossflow one not only for its inherent advantage but because it results in a higher speed of the water in the tube and this will reduce resistance to heat transfer to the inner walls of the tubes. CONCLUSIONS The Seawater Greenhouse is a development that allows the cultivation of crops on arid coastlines. It produces its own freshwater and provides an environment in which transpiration is minimised. The process relies on condensation of moisture from humid air. This requires a condenser device having a large surface area (of the order of 1000 m 2 for a Greenhouse covering roughly the same area). This has prompted us to develop a low-cost and efficient condenser (the Watermaker) made from polythene tubes. The coefficient of mass transfer for the Watermaker, expressed per m 2 of surface area, is about 50% greater than for the metallic tube-and fin type condenser previously used, even though the gaps through which the air flow are wider. This could be because the flow is more turbulent in the Watermaker condenser, leading to better mixing. The performance of the condenser has been compared to standard theory for heat exchangers. Reasonable agreement has been obtained therefore giving some guidance on how to optimise the design. Nevertheless, a rigorous method for the design of condensers of this type appears to be lacking. In the future, it may be possible to devise such a method based on fundamental physical principles that other authors have studied in the context of dew and aerosol formation. 6

7 NOMENCLATURE c d l n Nu Pr Re a dimensionless constant diameter of tubes length of tubes a dimensionless constant a dimensionless constant Nusselt number Prandtl number Reynolds number REFERENCES Barrett J.C. and Baldwin T. J.: Aerosol Nucleation and Growth During Laminar Tube Flow: Maximum Saturations and Nucleation Rates, J. Aerosol Sci., volume 31, (2000). Beysens D.: The formation of dew. Atmospheric Research, volume 39, (1995). Clement C.F: Aerosol formation from and heat and mass transfer in vapour-gas mixtures, Proc. R. Soc. Lond A, volume 398, (1985). Isachenko V.P., Osipova V.A. and Sukomel A.S.:Heat Transfer, MIR Moscow (1987). Vörösmarty C.J., Green P., Salisbury J. and Lammers R. B.: Global Water Resources: Vulnerability from Climate Change and Population Growth, Science volume 289, (2000). 7