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Liquid Desiccant Air Conditioning

This section presents a summary of the literature surrounding liquid desiccant air conditioning. Section 2.3.1 describes the operating process of a liquid desiccant air conditioning system. The aim is to highlight the significant advantage of adopting liquid desiccant in a SOFC tri-generation system. Section 2.3.2 details the different contactor designs and desiccant working fluids used. The aim is to highlight the benefit of employing a membrane based contactor with a potassium formate working solution. Section 2.3.3 provides a review of different liquid desiccant based air conditioning systems, with particular reference to those operating on waste heat sources. Throughout Sect. 2.3, specific consideration to tri-generation system integration is provided.

Operating Process

As highlighted in Sect. 1.1 buildings consume significant quantities of energy, and thus they are a large contributor to CO2 emissions. HVAC systems are a major source of this energy use in buildings, accounting for around 50 % of total supplied energy (Perez-Lombard et al. 2008). Air conditioning is a key function within HVAC systems, and is widely used in a range of buildings such as homes, schools, supermarkets and sport centres. Although in many Middle East, Far East and American regions air conditioning has become the expected norm, it has more recently received growing use in European countries e.g. UK, Denmark and Germany. This is due to more frequent warm spells, improved building insula- tion/air tightness and the use of in-house heat generating appliances (Smith et al. 2011).

Currently, the air conditioning market is dominated by vapour compression systems (VCS) because they have good stability in performance, low cost, long life and reasonable electrical COP (COPei) of between 2 and 4 (Welch 2008). However, VCS make use of harmful refrigerants such as R-22, R-410A, R-134A, materials with high global warming potential (Ouazia et al. 2009), and consume significant quantities of electrical energy to drive the compressor. Owing to the fact that the most common form of electrical generation in the majority of counties is from the combustion of fossil fuels, VCS can be viewed as neither a sustainable nor efficient air conditioning option (Zhang 2006a). It is thus apparent, with an already high and continually growing global demand for air conditioning there is a need for alternative options that do not rely so heavily on fossil fuel derived electrical energy.

There are a variety of air conditioning systems which reduce the requirement of electrical energy and in place of this use thermal energy to operate. This thermal energy can be sourced from solar or waste heat applications, thus the associated CO2 emissions from the air conditioning process will be much lower than for the equivalent VCS. This is due to the reduction in electrical requirement and the consumption of waste heat for a useful process. A variety of thermally activated cooling technologies exist, including: vapour absorption, solid adsorption, ejectors and solid/liquid desiccants. Vapour absorption systems (VAS) replace the electrical driven compressor found in VCS with a heat driven absorber and generator, these act in combination as a thermal compressor in the system. Absorption systems have relatively low thermal COP (COPth), in the range of 0.5 in single effect cycles up to 1.2 in double effect cycles (Srikhirin et al. 2001; Welch 2008), which results in the intensive use of thermal energy. Furthermore, due to pressurised operation, the need for high temperature waste heat, expensive and corrosive chemical solutions, e.g., LiCl, LiBr, CaCl2, VAS are relatively large and complex, and this has limited their attraction to many users (Duan 2012) and to applications greater than 10 kW (Pietruschka et al. 2006).

Desiccant air conditioning systems utilise the capability of desiccant materials in removing moisture from an air stream by the natural sorption process. Desiccant systems can be categorised as either solid (adsorption) or liquid (absorption); both types have their advantages and disadvantages. Liquid desiccant systems have lower regeneration temperatures, greater dehumidification capacity and lower air side pressure drop—thus reduced fan power requirement. On the other hand, solid desiccant systems are compact, simple, less subject to desiccant carryover and corrosion. In this thesis liquid desiccant is employed as the thermally activated cooling technology in the tri-generation system. Table 2.2 provides a comparative summary between liquid desiccant and VCS technology.

The information summarised in Table 2.2 illustrates the significant advantages of using liquid desiccant air conditioning, particularly in high humidity applications where waste heat is available. Liquid desiccant air conditioning represents significant advantages compared to both VCS and VAS for efficient air conditioning and tri-generation system applications, these include:

  • • No harmful refrigerants or working fluids are required
  • • Reliable and simple, operate at atmospheric pressures
  • • Can be driven by low grade waste heat (45 °C upwards)
  • • Decoupling of latent and sensible heat removal. Thermal comfort requirements can be met over a large range of conditions in a low energy, efficient manner
  • • Effective at cooling capacities of less than 10 kW

A drawback of the use of liquid desiccant air conditioning is that its application and performance is more location/climatic specific compared with VCS and VAS. As demonstrated in the Chap. 3 parametric analyses the dehumidification and regeneration processes are largely influenced by the inlet air conditions.

Liquid desiccant air conditioning systems operate on an open cycle absorption principle and consist of three main components, shown in Fig. 2.4: regenerator, desiccant evaporative cooler and dehumidifier. A supply air evaporative cooler is optional. The system consists of three main flows of fluids, the liquid desiccant solution, water and air.

Figure 2.5a shows a property plot of the desiccant solution process and Fig. 2.5b shows a psychrometric chart of the air process. A simplified description of the operation of a liquid desiccant air conditioning system is provided below.

Outside (supply) air flows through the dehumidifier, coming into contact with strong cool liquid desiccant solution. Because the desiccant solution exhibits a vapour pressure and temperature lower than that of the hot humid air (D1), it

Table 2.2 Comparison between liquid desiccant and VC air conditioning systems (Mei and Dai 2008)

Vapour compression

Liquid desiccant

Initial investment

Similar

Similar

Operation cost

High

Up to 40 % saving

Driving energy source

Electricity, natural gas

Low-grade heat

Humidity control

Average

Accurate

Temperature control

Accurate

Average

Indoor air quality

Average

Good

Location specific

Slightly

Yes

System instalment

Average

Complicated

Energy storage capacity

Bad

Good

A stand-alone desiccant air conditioning system

Fig. 2.4 A stand-alone desiccant air conditioning system

a Property plot of desiccant process. b Psychrometric chart of air process (ASHRAE 2009)

Fig. 2.5 a Property plot of desiccant process. b Psychrometric chart of air process (ASHRAE 2009)

causes dehumidification and sensible cooling (A1-A2). The supply air can then pass through an optional evaporative cooling device (performs better with drier air) to lower its temperature further (A2-A3). This air is then supplied to the building application.

During the dehumidification process, the desiccant solution becomes weak and warm due to the moisture sorption process (D2). The solution needs to be re-concentrated so that it may be used again. After passing through the dehumidifier, the desiccant solution flows to the regenerator. In the regenerator, the weak and warm desiccant solution has heat applied to it to desorb the moisture added in the dehumidifier; resulting in a strong but hot desiccant solution (D3). In some applications heat may also be applied by heating the regeneration airstream. Following regeneration, the desiccant solution needs to be sensibly cooled to restore an effective dehumidification and sensible cooling potential. A low energy solution is to use an evaporative process. Evaporative cooling of the desiccant can either be a concurrent process alongside dehumidification, taking place on the other side of the dehumidifier exchanger wall, or an evaporative cooling tower can be employed to produce cooled water which transfers coolth to the desiccant solution in a plate heat exchanger. During this process, the desiccant solution is sensibly cooled without moisture addition. Following this the desiccant solution is strong and cool (D1), restoring its dehumidification and sensible cooling potential.

Desiccant based air conditioning systems are primarily powered, for the regeneration process, by thermal energy. If this thermal energy is from solar or waste heat sources, it can significantly reduce both the operating cost and associated CO2 emissions of the air conditioning process. A recent study by Kozubal et al. (2011) of a desiccant enhanced evaporative air conditioning system demonstrated a 30-90 % reduction in energy demand compared to an equivalent VCS. Liquid desiccant air conditioning is a well suited technology for tri-generation system applications, particularly on a domestic building scale. This is due to their efficient use of low grade thermal energy, minimal electrical requirements and operation at atmospheric pressure.

Next, Sect. 2.3.2 summarises the different contactor designs employed in liquid desiccant air conditioning systems.

 
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