Numerical and experimental investigations of solar-powered multi-stage flash and humidification-dehumidification desalination using volumetric absorption

Abstract

Water is the fundamental need of every human being, but its shortage is affecting large number of developing countries which are home to more than half of the world’s population. Solar-energy driven humidification-dehumidification (HDH) desalination technology is well-suited for those water-stressed regions where demand for fresh water is decentralized and small-scale (5 to 100 m3/day). These regions are sometimes economically backward, having limited technical support and sufficiently far away from pipeline distribution facilities but at the same time may an advantage of receiving high incidence of solar energy. In a typical water-heated HDH cycle driven by solar energy, commercially available surface absorption-based solar thermal collector such as flat plate or evacuated tube is used to heat the feed water (seawater or brackish water) to a desired temperature called top brine temperature (TBT) having a maximum limit of around 90oC. However, heating seawater having very high salinity in the range of 35-55 g/L directly in these types of solar collectors to such high temperatures results in the precipitation of scale-forming compounds from seawater or feed water on their heat transfer surfaces which is undesirable. Scale formation impedes heat transfer which leads to reduced performance of the overall cycle, causes unscheduled shutdowns and premature equipment failure. In view of these issues, a novel water-heated HDH desalination cycle having closed-air and open-water (CAOW) configuration is proposed in which a nanofluid-based direct absorption solar collector (DASC) which does not have any metallic component is used to collect the incident solar energy and transfer it to the feed water via a counter-flow heat exchanger such that scale formation (sludge formation also) is limited only to the heat exchanger which can be cleaned easily. In DASCs, solar energy is directly absorbed by a stable nanoparticles dispersion, popularly known as ‘nanofluid’, which is having the high solar energy absorption capability and due to this fact these collectors also offer the higher thermal performance (up to 5%) as compared to their surface absorption-based counters parts. The DASCs can also be employed to power large-scale thermal desalination facilities such as MSF (multi-stage Flash), MED (multi-effect Evaporation) etc. The performance of these processes increases with increase in TBT which is normally limited to 100oC due to scale forming potential of seawater at high temperatures. In the present work, as a very first step a mathematical model solved using finite difference implicit method is prepared for DASC. The results of the model conclude that thermal performance of DASC is largely affected by thickness of nanofluid layer inside DASC and concentration of nanoparticles inside nanofluid. Further, the mathematical model for DASC is coupled with the mathematical model for brine-recirculation (BR) multi-stage flash (MSF) desalination cycle to study the thermal performance of the overall cycle evaluated by gained output ratio (GOR). It is concluded that GOR of the overall cycle is maximized at an optimum thickness of nanofluid layer inside DASC and as well as at optimum concentration of nanoparticles inside nanofluid. The GOR of the cycle was found to be linearly increasing with length of the collector and incident flux on the collector. The GOR obtained for this cycle has been compared with the GOR of the BRMSF system driven by a parabolic trough collector (PTC) and it is found that under identical conditions, DASC driven BR-MSF gives relatively 11% higher GOR as compared to PTC driven MSF system. Similar mathematical model is also prepared for DASC driven CAOW and water-heated HDH cycle to study the effect of various parameters related to both solar collector and HDH system on thermal performance of the overall which is calculated in terms of GOR and mass flow rate of the distillate produced. The thermal performance of the overall cycle is dominated by the top brine temperature which gets affected by the above-mentioned parameters of the solar collector, mass flow rate ratio and effectiveness of the humidifier and dehumidifier. A transport model for CAOW and water-heated HDH cycle is also prepared which quantifies the actual thermal performance of the HDH cycle by considering the fixed sizes of its principal components such as humidifier (counter-flow packed-bed cooling tower) and dehumidifier (finnedtube heat exchanger). It is concluded that at some fixed operating conditions such as mass flow rate ratio, top brine temperature etc, there exists an optimum size of the humidifier and dehumidifier at which their effectiveness is maximum. Finally, it is concluded that such models are helpful to thermally design a HDH system for a required GOR and distillate production rate. The transport model for this cycle is validated using the very basic thermodynamic model prepared for the same cycle. In addition to this, the results of the transport model for this cycle are validated against the results of the experiments performed on the same cycle and results from both studies are in close agreement with each other. Proof-of concept experiments were performed only on the CAOW and waterheated HDH cycle to study its thermal performance as a function of mass flow rate and TBT. The thermal performance of the cycle is measured in terms of effectiveness of both humidifier and dehumidifier, mass flow rate of distillate produced and GOR. At mass flow rate ratio of 2.34, average top brine temperature of approximately 62oC and average feed water temperature of 22oC, the installed system has been found to produce 5.2 litres/hr of fresh water with a GOR of 0.81.