Micro tri-generation system for indoor air conditioning in the Mediterranean climate
Introduction
Air-conditioning of buildings is a promising application of co-generation systems during summer. For this purpose thermally driven equipment to supply cooling driven with the co-generator waste heat has to be employed. The most widespread technology used for this purpose is based on heat driven water chillers such as absorption chillers (e.g. with the material pair lithiumbromide–water) or adsorption chillers. Water chillers are used in combination with different techniques to purge cooling loads from the rooms such as e.g. fan-coil systems, chilled ceilings or centralized air handling units (AHU). However, in order to treat latent loads, air has to be cooled below the dew-point when chilled water systems are used. Thereby the air is cooled far below the temperature level needed for comfortable indoor conditions and consequently the chiller works at a COP lower than if employed for sensible cooling, i.e. temperature control, only.
An alternative to treat latent loads by cooling air below the dew-point is the direct treatment of ventilation air in an open sorptive cooling cycle, also referred to as desiccant cooling system. In such a cycle air dehumidification is realized using a sorptive component such as a sorptive wheel. Additionally, a temperature decrease can be achieved by combination of the sorptive dehumidification with either direct, indirect or combined (direct + indirect) evaporative cooling.
However, the standard desiccant cooling cycle, which is for instance installed in temperate climates like Central Europe, is not able to cope with the conditions of warm and humid climates such as for instance in the coastal zones of the Mediterranean countries. Therefore application of desiccant technology in such climates using sorptive rotors requires specific configurations. In the framework of the project MITES (“Micro Tri-generation System for Indoor air conditioning in the Mediterranean Climate”), a project supported by the European Union, a novel configuration of an open cooling cycle based on sorptive rotor technology has been developed. This heat driven air handling unit receives its driving heat from a motor co-generation unit and is specially designed for weather conditions with high humidity ratios of the ambient air. A pilot system has been installed in fall 2003 at the client building of the gas utility of the municipality of Palermo (AMG) in Sicily/Italy and the system is commissioned and operated with accompanying monitoring during 2004.
Section snippets
Desiccant air handling unit configuration
As a first step of the project different configurations of desiccant air handling units were compared in order to identify the configuration which is able to provide desired supply air conditions with a minimum of energy consumption. However, before the different new designs are presented and compared, the standard desiccant cycle as used in temperate climates is described in detail in order to show the general operation principle. Based on this cycle different modifications were made in order
Cycles adjusted to humid climates
For all studied cycles the same boundary conditions, i.e., temperature and humidity values of ambient air, supply air to the building, return air from the building and regeneration air to regenerate the sorption material were assumed. These values are shown in Table 1. The following modified cycles which all use cooling coils in addition to the sorptive wheel were studied regarding their energy performance:
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Standard cycle with a cooling coil added behind the heat recovery wheel on the supply air
Design of the complete system
After the design of the desiccant air handling unit the design of the complete system consisting of the co-generation unit, the air handling unit, the compression chiller and the building related components has been made, Based on a detailed cooling load calculation it became obvious that not all cooling loads can be covered by the air handing unit. Therefore a fan coil system is operated in addition which is supplied with chilled water from the compression chiller as well. A scheme of the
System simulation
In order to be able to calculate an energy balance of the entire system for a whole year a simulation model of the complete system has been set up. Goal of the simulation tool is also to investigate different control strategies and to develop a tool for design of similar systems in the future. For this purpose the simulation platform TRNSYS has been used (see TRNSYS [3]). Specific sub-routines for modeling of the co-generation unit and the desiccant air handling unit with two cooling coils have
Conclusions
Different design options of air handling units using sorptive wheels for climates with a high level of environmental air humidity are presented and compared. Based on the comparison of the configurations at design conditions, a system using a combination of sorptive wheels and cooling coils was selected. An electricity saving of more than 30% compared to a conventional air handling unit is expected by using waste heat of a co-generation unit. A pilot system has been installed at the building of
Acknowledgements
We gratefully acknowledge support from the European Union for the MITES project.
References (3)
- M. Motta, Thermodynamic design and optimisation of solar assisted desiccant cooling cycles for Italian climates, PhD...
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