Thermal regeneration

Thermal regeneration is where heat is introduced into the regenerator by heating the air and/or the desiccant (Liu et al. 2009). The heat sources investigated for thermal regeneration have included heat generated by solar energy, heat from condensers of vapour compression systems, and waste heat from power engine generators, for example.

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2.3.1.1 Solar energy as the heat source

Regeneration driven by solar thermal energy has been investigated extensively because it is an energy source with very low running cost. The use of solar energy to regenerate liquid desiccants can be categorised as either direct or indirect heating. Direct heating means that solar collectors are used to directly heat liquid desiccants for regeneration, whereas indirect heating uses solar collectors to heat an intermediate working fluid that is then used for liquid desiccant regeneration.

Direct heating

Peng and Zhang (2011) used a solar collector as the thermal regenerator for liquid desiccants in a solar LDAC system. The forced flow solar collector had an inclined flat black surface that was used to absorb solar energy (Fig. 2.3). These solar collectors were designed with either single or double glazed covers which reduced heat losses and prevented the liquid desiccant from being contaminated by foreign substances from outside environment. A weak liquid desiccant passed over the black surface as a thin liquid film which was then heated by the solar collectors. Water that evaporated from the liquid desiccant was removed by a forced air stream. An analytical model for the performance calculation of proposed solar collector regenerator was developed and validated with experimental data.

Fig. 2.3 (a) Schematic diagram and (b) Cross-section view of solar collector (Peng and Zhang 2011).

Another example of using solar collectors to directly heat the liquid desiccant was presented by Katejanekarn et al. (2009). Fig. 2.4 shows the experimental setup

21 used. Here, liquid desiccant was heated in a solar collector, where the water evaporated and was then removed by scavenging air.

Fig. 2.4 Schematic of the LDAC system proposed by Katejanekarn et al. (2009).

Jian (2014) also used a solar collector to directly heat the liquid desiccant solution, see Fig. 2.5. The liquid desiccant was heated in a solar collector and then sprayed into the regeneration tower where the water that evaporated from the solution was removed by scavenging air. The effects of the inlet temperature and humidity of air, the mass flow rate of scavenging air, and the inlet temperature of a weak solution on the regeneration capacity were experimentally studied.

22 Fig. 2.5 Solar regenerated LDAC system by directly heating the liquid desiccant proposed by Jian (2014), where 1-direct evaporative cooler, 2-air dehumidifier, 3- solution tank, 4-spiral-plate heat exchanger, 5-solution tank, 6-solar collector, 7- cooling tower, 8-regenertator, 9~11-solution pump, 12~13-fan, 14-thermocouple.

Indirect heating

Keniar et al. (2015) used parabolic solar collectors for liquid desiccant regeneration, as shown in Fig. 2.6, where the solar collectors were used to heat an intermediate working fluid stored in a tank, which was then used to supply the heat in one heat exchanger to regenerate the liquid desiccant. The humidity removal capacity was calculated through mathematical modelling and was validated with experimental data.

23 Fig. 2.6 Schematic of LDAC system with parabolic solar collectors (Keniar et al.

2015).

Armanasco et al. (2015) used solar collectors to regenerate a liquid desiccant by indirectly heating in a large scale (>20 kW) liquid desiccant solar cooling plant, as shown in Fig. 2.7. Here the solar collector was used to heat a mixture of water and glycol, which was then used to heat water in the hot water storage, and the stored hot water was then used to heat the liquid desiccant for regeneration. A performance analysis was carried out on experimental results of this installed plant.

24 Fig. 2.7 Schematic of the large-scale liquid desiccant solar cooling plant (Armanasco

et al. 2015).

Xiong et al. (2008) used a solar collector to indirectly heat LiBr and CaCl2 liquid

desiccants for regeneration. As Fig. 2.8 shows, there are two stages of

dehumidification in this proposed system, using solutions of LiBr and CaCl2,

respectively. The solar collector was used to heat the water used for heating LiBr in a

heat exchanger, and then for heating CaCl2 solution in another heat exchanger. The

25 Fig. 2.8 Two-stage liquid desiccant dehumidification system (Xiong et al. 2009).

The disadvantage of using solar energy for thermal regeneration is that it is highly dependent on weather conditions and may not continuously provide sufficient cooling and dehumidification, especially during the night.

2.3.1.2 Waste heat from condensers as a heat source for thermal regeneration

The use of waste heat from the condensers of conventional vapour compression systems for liquid desiccant regeneration has been reported in a number of studies. A feasibility study and performance analysis of a LDAC system using hot air from the condenser of a vapour compression system to regenerate liquid desiccants was studied numerically by Yin et al. (2011), as shown in Fig. 2.9. They showed that it is technically feasible to regenerate liquid desiccant using hot air from the condenser, and suggested that the scavenging air should be around 65°C for LiCl liquid desiccant.

26 Fig. 2.9 An LDAC system using hot air from the condenser for regenerating liquid

desiccant (Yin et al. 2011).

In a hybrid LDAC system (Fig. 2.10) proposed by Yamaguchi et al. (2011), the heat released from the condenser would be used to heat the scavenging air and liquid desiccant solution entering the liquid desiccant regenerator. The performance evaluation was conducted by both experiments and mathematical calculations.

Fig. 2.10 A hybrid LDAC system using the condenser to heat both the scavenging air and liquid desiccant (Yamaguchi et al. 2011).

Mohan et al. (2015) experimentally investigated the performance of a hybrid liquid desiccant vapour compression air conditioning system, as shown in Fig. 2.11,

27 where the ambient air was first heated by the condenser and then supplied to the regenerator to regenerate the liquid desiccant.

Fig. 2.11 An LDAC system with regeneration driven by waste heat from the condenser to heat the scavenging air (Mohan et al. 2015).

In an LDAC system proposed by She et al. (2015), as shown in Fig. 2.12, liquid desiccant was regenerated by using the condenser to heat the scavenging air and heat the weak solution in a solution-to-refrigerant heat exchanger before the condenser. To improve the heat utilisation efficiency of the condenser for regenerating liquid desiccant, She et al. (2015) analytically investigated how the heat distribution ratio (the percentage of amount of heat from the condenser used for heating air and liquid desiccant solution) affected the scavenging air and liquid desiccant solution and discovered that a system that used all or part of the available heat to heat the solution performed better than a system where all the heat was used to heat the scavenging air, if the condensing temperature was relatively low or the concentration of the solution was relatively high.

28 Fig. 2.12 An LDAC system with regeneration driven by waste heat before the

condenser, and from the condenser, as proposed by She et al (2015).

Zhang et al. (2013) compared heating the weak solution with heating the scavenging air through the heat released by the condenser in heat pump driven liquid desiccant systems and discovered that the system performed better when the weak solution was heated for regeneration.

2.3.1.3 Using heat from electric power generation engines

The waste heat from an electric power generation engine has also been used to regenerate liquid desiccants. Gao et al. (2008), for instance, proposed the use of waste heat from diesel engines for a cooling, heating, and power system to regenerate liquid desiccants. It was claimed that a liquid desiccant system can supply 42% of the cooling and reduce the chiller energy consumption by 9.5%. A combined heat and power system consisting of a natural gas engine generator and a liquid desiccant dehumidification system was designed and installed in a four-storey educational office building by Nayak et al. (2009). Here, waste heat from the exhaust gases and jacket water was used to regenerate the liquid desiccant; from which it was found that the primary energy consumed by the integrated heat and power system was 4.2%

29 less than a conventional power plant. In a laboratory, a combined cooling, heating, and power (CHP) system was developed by Fu et al. (2009). In summer, waste heat from the jacket water was used to regenerate liquid desiccant, and under certain operating conditions, this combined CHP system could achieve efficiencies up to 90%. An innovative natural gas CHP system combined with a gas-fired internal combustion engine and a LiCl liquid desiccant cooling system was developed by Badami and Portorato (2009). In the proposed tri-generation plant, heat from the flue gases and cooling water for the engine was recovered and used to regenerate the liquid desiccant. This proposed combined CHP system is a possible alternative to traditional cooling technology.

Although heat from electric power generation engines can regenerate liquid desiccants and improve the performance of combined CHP systems, the engine generator source used for regeneration might have a location limitation and the regeneration temperature is still relatively high.