**2. Solar thermal energy storage and desalination application**

 Thermochemical materials (TCM) have many advantages over the other materials such as high heat storage density and low heat leak. Once the reactant leave the thermochemical materials, the enthalpy remains same and it help to achieve the state of energy charging. Subsequently, the discharged energy is utilized while the material remains stable. In the past, a lot of studies were carried out on heat pump using different TCM materials [12–14]. The selection of TCM materials for different application is based on many elements such as (i) heat storage temperature, (ii) heat releasing temperature,

**Figure 3.**  *MgO thermal energy storage system operation.* 

(iii) heat storage density, and (iv) material stability. The magnesium oxide (MgO) is most suitable for thermal heat storage as compared to other materials due to its high density and stability. Many researchers published data on MgO thermal heat storage and its performance improvement [15, 16].

 The dry MgO reacts with water (hydration) to become hydrated Mg(OH)2. The hydration is a exothermic reaction and generates 81 kJ/mol. During dehydration of Mg(OH)2 it becomes MgO through a reverse process at 350–500°C from solar collectors and high temperature vapor are utilized as a heat source. It can be noticed that MgO as an energy storage material produce heat during day as well as night time. The hydration process at night and dehydration process at day with solar energy can produce sufficient heat energy to operate the desalination cycle.

 The principle of this heat pump is shown in **Figure 3**. The heat pump consists of a magnesium oxide reactor and a water reservoir. In heat storage mode magnesium hydroxide (Mg(OH)2) is dehydrated by surplus heat (Qd) at Td from sun. The generated vapors are condensed at the reservoir at TC and the condensation heat (Qd) of the vapor is used for desalination cycle at day time. The hydration of magnesium oxide proceeds in the reactor by introducing the vapor, and a hydration heat output (Qh) at Th is generated to operate desalination cycle at night. Thermal drivability, which does not require mechanical work, is one of the advantages of the heat pump. The environmentally friendly and economical nature of the reactants is also advantageous. This type of heat pump is able to store heat at around 350°C through Mg(OH)2 dehydration and to transfer stored heat at temperatures between 110 and 150°C through MgO hydration. The solar thermal energy storage and 24 h delivery around 100°C is best suitable for sustainable desalination processes [17–19].

The renewable energy (RE) driven desalination processes are already commercialized but at low scale due to some operational complexity. **Table 3** summarized


*\* Cost is estimated based on plant capacity more than 1000 m3 /day [20]. \*\*Refs. [21–29].* 

#### **Table 3.**

*RE driven desalination technologies and water cost.* 

*Desalination with Renewable Energy: A 24 Hours Operation Solution DOI: http://dx.doi.org/10.5772/intechopen.84944* 

the major renewable desalination plants operated in the world and estimated cost of water production.

 It can be noticed that thermal desalination processes are most favorable option with solar thermal energy operation. The most efficient thermally driven multi effect desalination (MED) system recently investigated to overcome its conventional operational limitations. The numbers of stages in a MED is controlled by top brine temperature (TBT) and lower brine temperature (LBT). The TBT typically 70°C is restricted by soft scaling components in the feed water and the LBT is controlled by ambient condition due to water cooled condenser to condense the last stage vapors. The MED system can be more efficient if these two limitations cab be removed to increase number of recoveries. Researchers found that TBT can be increased to 125°C by inducing nano-filtration (NF) prior to introduction the feed into the system. This NF process helps to remove soft scaling components and prevent scaling and fouling on the tubes of evaporators. The inter stage temperature and the last stage operating temperature limitations can be overcome by hybridization with adsorption cycle. AD cycle can operate below ambient conditions typically as low as 5°C due high affinity of water vapors of adsorbent (silica gel). MED last stage temperature can be lower down to 5°C by introducing the AD at downstream. The proposed hybrid MEDAD system with TES will be the best choice for sustainable water supplies.
