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Simulation and analysis of a solar-assisted trigeneration system

Adsorption Chiller

Main positive and negative aspects of sorption cooling are described in this section, with respect to the studied system application. Besides, reasons why an adsorption cycle has been preferred to an absorption one, for recovering ORC rejected heat, are provided.

Nowadays, there is a growing interest in the recovery of low-temperature thermal energy that is typically wasted, which is leading to further possibilities of usage of these limited quality heat sources. According to the EPA, just in the United States it is estimated that the potential for waste heat recovery could substitute approximately 9% of the total US energy consumption [1.31].

Most common sources of rejected heat are prime movers cooling system and a number of industrial processes. Sources temperatures usually ranges between 60°C and 100°C and thus, in most cases can only be exploited for heating purposes.

Sorption cooling through adsorption and absorption chillers, offers a possibility of effective utilization on a large scale of this energy for chilled water production. Chilled water demand for air conditioning and for industrial cooling processes is constantly growing, indeed, with large amounts of electricity consumed by conventional air conditioners.

Sorption refrigeration technologies are thermally driven systems, making use of a heat source and a sorbent to produce a cooling effect. The sorbent can be either solid in the case of adsorption systems or liquid for absorption systems.

Sorption chillers systems are often used in combination with solar collectors (71% of current installations) to perform the so-called solar cooling and are expected to compete with traditional air conditioning systems in the next 20-30 years [1.32].

Concerning an absorption refrigeration cycle, the process is nearly identical to that of a mechanical refrigeration, but no compressor is used because the regeneration of the refrigerant fluid is driven by a medium-temperature heat source. The refrigerant is absorbed by the coupled fluid, the blend is then pumped to the generator in which the refrigerant is desorbed and then evaporates in order to perform the cooling effect. Absorption chiller normally use H2O/LiBr and NH3/H2O as refrigerant/absorber working pair [1.33].

Adsorption cycles use solid sorption materials, such as silica gel or zeolite, instead of liquid refrigerant and water as refrigerant. They have an inherent robust structure, requires little maintenance and present higher efficiency than absorption chillers at low operating temperatures. On the contrary, they are generally heavier and needs higher specific investment costs, especially in large power ranges. That is because adsorption is a spontaneous cyclical process and requires large heat transfer surfaces (i.e. multiple adsorbent beds) to provide a continuous capacity during operation.

Active research regarding adsorption chillers is being carried out in many countries ([1.34], [1.35]) and commercial adsorption units are available in capacities of up to a few megawatts.

Since the trigeneration system which is matter of study have a limited waste heat capacity and low source temperature, an adsorption chiller has been selected for cooling production.

There also some other benefits related to adsorption chiller employment.

Thermally driven chillers based on absorption process have been effective but with high operation and maintenance costs: the LiBr solution used in absorption chillers tends to solidify within the system, in case of too high or too low regeneration temperature, or when conditions change too rapidly for the system to adapt; for this reason, many installations of absorption units require lot of careful periodic maintenance.

On the contrary, adsorption chillers does not make use of Li-Br, ammonia and not even CFC or freons as in other refrigeration cycles. This means no potential for hazardous material leaks, aggressive corrosion, and null ODP.

Besides, adsorption chillers have very few moving parts, can operate continuously and have very short start up and shut down procedures.

Questo brano è tratto dalla tesi:

Simulation and analysis of a solar-assisted trigeneration system


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Informazioni tesi

  Autore: Simone De Luca
  Tipo: Tesi di Laurea Magistrale
  Anno: 2015-16
  Università: Università degli Studi di Trieste
  Facoltà: Ingegneria
  Corso: Ingegneria Meccanica
  Relatore: Marco Manzan
  Lingua: Inglese
  Num. pagine: 176

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