Reverse vapor compression

Reverse vapor compression (RVC) produces power by exploiting the vapor pressure difference between two solutions with different salinities. As a matter of fact, it does not require semipermeable or cation/anion-exchange membranes as PRO or RED technologies, and, therefore, it results much less demanding in terms of water filtration and maintenance. This appealing characteristic is however penahzed by a really low-power density, which entails huge devices for a relatively limited power production with costs of electricity strongly affected by the cost of the components. 

According to the function of Py T) for both saltwater and freshwater, the pressure difference is calculated and the maximum theoretical power results as 

For an ideal gas, vAp = cpyAT, where cpy is the specific heat at constant pressure. Thus, Eq. 9.15 may be rewritten as 

ATy jjjax and Apv,max representative for the same phenomena and can be indifferently used to calculate the maximum theoretical specific power l th,max-Nevertheless, the continuous exploitation of such a pressure difference is not trivial since the process evolves spontaneously to an equilibrium condition. Vapor formation causes the liquid phase to cool down, with a reduction in vapor pressure at the freshwater chamber. In the other chamber, vapor condenses with an increase of temperature and vapor pressure. If no heat is provided to the freshwater chamber and extracted from the saltwater chamber, the small pressure difference that drives the process rapidly vanishes, causing an interruption in the vapor flow. 

The solution proposed in literature considers heat transfer from the salt to the freshwater chamber in order for the system to operate continuously. As a result, the theoretical pressure drop cannot be exploited, unless maintaining a temperature difference between the two sides of the conducting wall that divides the two chambers  

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The vapor pressure of an aqueous solution depends on its salt concentration. Curves of the saturation pressure as a function of temperature are reported in Figure 9.10 for freshwater, seawater, and brine with a salinity of 0, 3.45%, and 28%, respectively [28]. Vapor pressure increases quickly with temperature, even though the trend is affected by salinity. Considering solutions at the same temperature, an extremely small vapor pressure difference can be appreciated between freshwater and seawater, but a wider gap can be observed with reference to a high-salinity brine. The reverse vapor compression technology exploits such a pressure difference. 

Nevertheless, there is a long way to go before these systems can reach a commercial application level. There are two main issues. Power density (specific to membrane area unit) attainable by the current membranes (approximately 1 W/m ) is too low to make the technology cost-effective. However, the development of membranes for a specific purpose has just been started and significant improvements are expected in the next future in terms of performance, durability and cost. The second main issue is fouling caused by particles entrained by the streams contacted to membranes, which has to be controlled by expensive and possibly polluting water pretreatment processes. The latter problem is definitely avoided by the other two alternatives proposed, reverse vapor compression and hydrocratic generator, which on the other hand have not yet proved their technical feasibility. 

All commercial types of processes, with the exception of freezing, namely, distillation, reverse osmosis and electrodialysis, are being applied in the above units with various kinds of distillation processes being used for seawater desalting. Two of them, horizontal tube multieffect distillation and vapor compression units were developed and manufactured locally by the Israel Desalination Engineering Ltd. Recently, two small RO units with a combined capacity of approx. 100 cu. m/day were also used to desalt seawater. The main aim of these units is to test and demonstrate the feasibility of this new technology. 

Vapor compression uses the reverse principle of multieffecl distillation. As a vapor is compressed, its temperature and pressure increase. The compressed vapor can be used instead of fresh steam as a heat source at the high-temperature end of the distillation process. 

So far only the energy requirement for a process in the form of work has been considered. Freezing, vapor compression, and reverse osmosis processes are examples of processes that require a work input. There are, however, other important processes, such as multiple-effect evaporation and flash evaporation, for which the energy input is in the form of heat. How does one relate the energy requirement of these processes to the minimum work of separation One method is to convert the heat requirement to a work equivalent by means of the Carnot cycle. If T is the absolute temperature of the heat source and T0 the heat-sink temperature, then one can use the familiar relation... 

The effectiveness of a refrigeration cycle is measured by its coefficient of performance. For given values of Tc and TH, the highest possible value is attained by the Carnot refrigerator. The vapor-compression cycle with reversible compression and expansion approaches this upper limit. A vapor-compression cycle with expansion in a throttle valve has a somewhat lower value, and this is reduced further when compression is not isentropic. The following example provides an indication of the magnitudes of coefficients of performance. 

Seawater or brackish water is used for process applications or as potable water when fresh water is scarce. Six techniques are used for desalination. Five are evaporation processes multiple-effect thermocompression mechanical vapor compression once-through multistage flash and multistage flash with brine recirculation. The sixth process, reverse osmosis, uses membrane technology for desalination. 

Seawater Supply Seawater required per unit of water produced varies from 2 for mechanical vapor compression to 3 for reverse osmosis and 8-10 for the other processes. Seawater requirements may dictate more or larger equipment. 

Maintenance requirements could be listed in the following order, from lowest to highest maintenance thermocompression once-through multistage flash multistage flash with brine recirculation reverse osmosis multiple-effect evaporation mechanical vapor compression. Several factors are important ... 

There are several types of heat pumps that are currently feasible for industrial applications, including mechanical vapor-compression systems, closed-cycle mechanical heat pumps, absorption heat prnnps, heat transformers, and reverse Brayton-cycle heat prnnps. 

If the process of water vapor compression is described like that for an ideal gas undergoing an adiabatic process, then the reversible work required for the compression is given by that for an isentropic process ... 

To reduce the energy required for a distillation process with almost pure water as overhead vapor, a heat-pump arrangement is proposed. The column is operating at atmospheric pressure. The reboiler needs a vapor condensing at 150 °C. The overhead water vapor should be compressed by a reversible adiabatic compression process such that it can be tbe condensing vapor in tbe reboiler. Determine the heat delivered by the condensing vapor per pound of the vapor calculate the ratio of the heat delivered in the reboiler to the work required in the reversible compressor. Use tbe steam tables from Smith and Van Ness (1975) or any other suitable source. (Ans. 1020 Btu/fb (567 cal/g) 7.254.)... 

Refrigeration systems are important in industrial and home use when temperatures less than the ambient environment are required. Of the several types of refrigeration systems, the most widely used is the vapor-compression refrigeration cycle. It is essentially a Rankine cycle operated in reverse, where heat is absorbed from a cold reservoir and rejected to a hot reservoir. Due to the constraints of the second law, this process can be accomplished only with a concomitant consumption of power. 

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