Liquid Desiccant Based Tri-generation Systems

This section will examine tri-generation systems employing liquid desiccant air conditioning technology.

A system employing an ICE and VAS is provided below; the work illustrates the benefits a liquid desiccant air conditioning system can bring to a tri-generation system. Liu et al. (2004) investigated a hybrid tri-generation HVAC system in a multi-storey demonstration building, employing an ICE, liquid desiccant and vapour absorption technology. The investigation highlighted issues with conventional tri-generation systems that use a straight VAS, these include: (1) low grade waste heat from the prime mover is used directly to drive the cycle, leading to low energy performance and thus a COPth of 0.7 or less, (2) how to match electrical and thermal loads of the building and lengthen the operating hours of the tri-generation system (thus improving benefits from the system), and (3) dehumidification in a VAS is achieved by cooling the air below its dew point. However, because the supply air requirement is 7-12 °C some re-heat is needed. This lowers the COP and can also cause health problems as the condensed water makes the coil surface a hot bed for bacteria. However, the authors found a way of avoiding the above problems when using a hybrid system consisting of liquid desiccant, VAS and thermal/desiccant storage. The findings are as follows: (1) the regeneration of liquid desiccant does not need high grade heat, only 70-80 °C, thus COPth improvements of ~1.2-1.3 can be achieved (2) a thermal/desic- cant storage tank can be used to match differences between thermal supply and demand, and (3) the liquid desiccant system will remove the latent loads, therefore avoiding the need to cool then re-heat. A simulation was carried out to analyse the performance of the building with the different system configurations i.e. with and without liquid desiccant. Results show that CO2 emissions are reduced by about 40 % and energy storage plays an important role in the hybrid liquid desiccant system.

Qiu et al. (2012) have carried out an experimental investigation of a liquid desiccant system driven by the flue gases of a biomass boiler. The liquid desiccant system consists of two parts; first, the dehumidifier and secondly an

IEC. The desiccant dehumidifier used was of a direct contact counter flow falling film design, using a potassium formate (CHKO2) solution. Internal cooling was provided from an evaporative device. The system was operated in autumn in Nottingham, UK and was found to be able decrease air temperature by 4 °C and relative humidity by 13 %. A total cooling capacity of the system was found to be 2.381 kW, the evaporative cooling device alone had a cooling capacity of 1.049 kW, illustrating the dehumidifier plays a significant role in supply air conditioning by reducing the airs enthalpy. Electrical consumption for fans and pumps totalled ~200 W; therefore the desiccant cooling systems COPei was in the range of 11-12, an encouraging value. It was found that the heat from the flu gas was insufficient to heat the desiccant solution to the required 40 °C, only 30 °C was reached, as a result future work will look at improving the flue gas heat exchanger, leading to improved performance from the liquid desiccant system. This work has proved that desiccant systems are effective cooling devices in waste heat scenarios, however issues such as waste heat recovery effectiveness need to be addressed in order to maximise operating performance. The experimental results confirm that cooling is a result of lowering both humidity and air temperature.

Jradi and Riffat (2014c) present a novel biomass-fuelled micro-scale tri-generation system with an organic Rankine cycle and liquid desiccant cooling unit. Potassium formate is used as the desiccant working fluid. Results from laboratory experimental testing show the system can provide 9.6 kW heating, 6.5 kW cooling and 500 W electric power. The dehumidification-cooling system attains a COPth of 0.86 and COPel of 7.7. The overall efficiency of the micro-scale tri-generation system is 85 %. The high COP results demonstrate the potential of using liquid desiccant air conditioning technology in the development of efficient and effective tri-generation systems, and as an alternative to conventional separate energy conversion systems.

Badami and Portoraro (2009) have carried out a performance analysis of a trigeneration plant based on manufacturer data of a gas fired ICE and liquid desiccant air conditioning system to supply cooling (210 kW), heat (220 kW) and power (126 kW) to a teaching building at the Polytechnic University in Torino, Italy. Both an energetic and economic analysis was completed. The energetic analysis showed that the system could provide an 18 % energy saving in winter (no cooling) compared to a grid electricity and separate boiler heat production system. For the summer season, when the liquid desiccant system is operated, a total primary energy saving of -12 % was reported, showing the system has a higher primary energy consumption than that of a traditional separate production system. Therefore the convenience to operate the system during the summer months was based on its economic profitability. Economically, the system was assessed to have a pay-back period of 6.8-7.6 years, have an internal rate of return of 9.711.5 % and a net present value after 15 years of 200-220 k€. Variation in these assessments is due to the consideration of various national incentives for CHP and tri-generation systems. The performance analysis has shown that the adoption of tri-generation systems based on liquid desiccant technology is feasible economically, however energetically some questions do arise and should be considered before application. A limitation of the study is the assumption that all energy outputs from the system are consumed, thus all energy is fully exploited. Further analysis including time varying energy supply and demand may cause a poorer examination of the system.

Badami et al. (2012) have also carried out an analysis based on manufacturer data, comparing a tri-generation plant composed ICE and liquid desiccant with a micro gas turbine (MGT) and VAS. Energetic analysis carried out according to EU Directive 2004/8/EC (EU 2004) has shown that primary energy savings (PES) for the ICE/liquid desiccant system is greater than the MGT/VAS; 16.8 % compared to 9.1 % respectively. However further analysis using the more suitable tri-generation primary energy savings (TPES) methodology proposed in the literature (Chicco and Mancarella 2007), showed lower savings of 12.4 and 4.1 % respectively.

Section 2.5.4 has presented a review of the literature surrounding liquid desiccant air conditioning systems adopted in tri-generation system applications. Liquid desiccant based tri-generation systems can offer environmental, economic and operational advantages over other heat driven cooling cycles. VAS are rarely used in tri-generation applications of less than 10 kW (Pietruschka et al. 2006), therefore liquid desiccant is a particularly well suited option for the development of a tri-generation system at a domestic building level.

It is clear that the use of liquid desiccant air conditioning in tri-generation system applications is feasible, and has been demonstrated across a range of building scales. Some clear benefits liquid desiccant air conditioning can bring to a tri-generation system application include:

  • • Effective use of low grade thermal energy; high COPth values reported.
  • • Removing the need to cool below the dew point to carry out dehumidification.

Careful consideration needs to be made so that the desiccant solution regeneration temperature can be achieved, this is critical for two reasons:

  • • A lower desiccant concentration will result in a reduced desiccant air conditioning system performance.
  • • Additional heating will reduce the potential savings of the system, thus impacting its feasibility as a replacement to a conventional separated system.

It is apparent that effective installation, optimisation and operation are all required in order to maximise the benefits attainable from the adoption of a liquid desiccant based tri-generation system. Otherwise issues such as an increase in energy demand seen in the work of Badami and Portoraro (2009) can result. Furthermore, the size and complexity of the liquid desiccant systems needs careful consideration, especially when integrated in a domestic application with other potentially large and complex equipment i.e. a fuel cell.

Next, Sect. 2.6 reviews the assessment methods used to evaluate the performance of tri-generation systems.

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