Desalination of salty water using vacuum spray dryer driven by solar energy

Highlights

• Theoretical analysis for desalination under reduced pressure aided by solar energy,

• New approach and design for a solar-aided desalination system under reduced pressure,

• New approach and design for solar energy collector using spherical-shaped system,

• Recycled latent heat of evaporation, waste heat from the vacuum pump, heat collected from the solar panels and droplet colour were considered in the theoretical analysis.

Abstract

This paper addresses evaporation under vacuum condition with the aid from solar energy and the recovered waste heat from the vacuum pump. It is a preliminary attempt to design an innovative solar-based evaporation system under vacuum. The design details, equipment required, theoretical background and work methodology are covered in this article. Theoretically, based on the energy provided by the sun during the day, the production rate of pure water can be around 15 kg/m²/day. Assumptions were made for the worst case scenario where only 30% of the latent heat of evaporation is recycled and the ability of the dark droplet to absorb sun energy is around 50%. Both the waste heat from the pump and the heat collected from the photovoltaic (PV) panels are proposed to raise the temperature of the inlet water to the system to its boiling point at the selected reduced pressure.

Introduction

Water and energy are fundamental necessities for life on Earth and to sustain the modern world. In many parts of the world, the control and exploitation of water and energy has driven economic development. In many other places there are shortages in fresh water and energy supplies. Drinking water of acceptable quality has become a scarce commodity not to mention unevenly distributed geographically worldwide [14], [24]. The World Health Organization (WHO) has estimated that lack access to drinking water may be an issue for more than a billion people [15]. The vast majority of these people are from undeveloped countries and/or living in rural areas where there is low population density. In remote locations it is very difficult and costly to install conventional clean water solutions, [25]. In many countries where there are shortages of clean and fresh water, there are many other water resources that have potential to be transformed. Such resources seawater, brackish/saline or contaminated groundwater, and coal seam gas water [26]. These kinds of water require an innovative technique of treatment that uses sustainable and low cost energy to produce clean water.

Desalination of saline water is known to be one of the most sustainable alternative solutions to provide fresh water. This resource can play a significant role in socioeconomic development in many developing countries such as Africa, Pacific Asia and countries in the Middle East [16] and Latin America. Desalination is a process in which saline water is separated into two parts: one that has a low concentration of dissolved salts (fresh water), and the other which has a much higher concentration of dissolved salts than the original feed water (brine concentrate), [27]. Reverse Osmosis (RO), a conventional desalination technology, produces brine (70–55% of intake flow) depending of feed water quality; the dissolved salt concentration of the brine varies from 50 to 75 g/L resulting in a much higher density than seawater (Hamawand et al., 2013). Desalination of salty water/seawater is expensive, mostly because of the energy required [17]. However, desalination is a growing field around the world where the needs for drinkable water are crucial [2]. All conventional seawater desalination techniques such as RO, thermal distillation such as multi stage flash (MSF), electro-dialysis, or their combinations consume a large amount of energy and they do not recover the salt eventually. These techniques may also cause air pollution due to the large consumption of energy derived from fossil fuels [19], [20]. Furthermore, the brine remains from these processes are huge and represent another potential environmental problem [18]. There is a potential for using algae to clean the water after amending it with some chemicals, however this has not been carried out experimentally [11]. Therefore, the utilization of renewable energy can be considered as one of energy sources of seawater desalination [3], [5], [17].

A complete separation of salt from salty water is something that cannot be achieved by many of the conventional methods. One conventional, most efficient and reliable method for complete separation is evaporation. Evaporation of water at atmospheric pressure requires large amounts of energy to raise the water temperature to the boiling point. In addition, at atmospheric pressure the evaporation rate is slow unless more vigorous heat is supplied. This problem can be overcome by carrying out the evaporation under reduced pressure where water can be evaporated at temperatures below its atmospheric pressure boiling point. Water evaporation under reduced pressure is energy efficient according to the laws of thermodynamics, and can be driven by low-grade thermal energy sources such as solar heat or process waste heat. While the evaporation under reduced pressure will accelerate the evaporation rate, one must be concerned with potential freezing problems [4]. Heat is required for water droplets to evaporate, heat is provided from the surrounding. Without supplemental heat there is a risk that the equipment parts get chilled and the remaining droplets freeze [1]. Carey et al. [21] conducted experiments under contract to NASA related to evaporation of water under vacuum. The set up consisted of 0.6 m³ environmental vacuum chamber and a 250 mL beaker with 25 mL of liquid water at a temperature of 20 °C. The liquid took approximately 150 min to evaporate under reduced pressure of 0.38 kPa. In this experiment no external heat was provided neither to the liquid or the chamber.

Valuable information on desalination of seawater using solar energy has been reported, however, the desalination of sea water using vacuum spray dryer has not been fully elucidated. This study presents an innovative design for evaporation of water using renewable energy from the sun and recycles the latent heat of evaporation. Also the waste heat from the pumps and collected heat from PV panels that provide the pump with electricity are suggested as another source of energy [7], [8].