Question

Based on the reading assignment and the textbook, answer the following questions:

a- The impact of increasing the number of effects in MED system on the performance ratio (PR)? And list the reasons (10 Points)

Answer 1

The high energy consumption of existing desalination technologies and transmission of water via pipelines remain among the main issues of the sector. While water production processes and technologies have experienced rapid advances in recent years, resulting in a dramatic reduction in energy consumption, the existing processes of seawater desalination are energy-intensive due to the irreversibility within various system components, such as in the evaporator of MED, due to vaporization, and in the pressurized membrane.This can be determined from the universal performance ratio (UPR), which is based on the primary energy of all existing commercialized desalination technologies.

The effects of different designs and/or control and operational parameters, such as condenser and evaporator temperature, number of beds, and operation strategies, on MED performance such as specific daily water production (SDWP), coefficient of performance (COP) of the cooling system and overall conversion ratio (OCR) has impact on PR. Additionally, different energy sources coupled with the MED system have been studied, the effects of different types of adsorbents on the performance of MED have been compared, the effects of combining MED with MED or other desalination technologies have been studied, and models and simulations have been conducted utilizing different isotherm models and validating the results with experimental data. The primary goal of the majority of these studies was to reduce the unit water cost and environmental impact, and to raise SDWP and the performance ratio.

Single and multi-stage evaporation desalination were the basic configurations for the desalination process, comprising a condenser, an evaporator, and a heat source. The performance ratio was less than one because the amount of vapor flow to the condenser was low. To improve the process and increase the vapor flow to the condenser, mechanical and thermal compression were introduced to the system. However, both required additional mechanical or thermal energy to compress the vapor and increase its amount. Single and multi-stage evaporation desalination were the basic configurations for the desalination process, comprising a condenser, an evaporator, and a heat source.

It can be concluded that maximum heat recovery occurred when the temperature difference between the two beds was minimal. This configuration increased the performance ratio of the system three to four times in proportion to the number of effects.