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Cost-Effective Solar AiR conditioning

Periodic Report Summary - CESAR (Cost-Effective Solar AiR conditioning)

Project context and objectives:

Summer electricity use is growing year upon year, mainly due to air conditioning (AC) demand. Electricity grids are increasingly experiencing problems during the hottest summer days, to deal with the increasing demand of electricity from air conditioning systems.

Absorption cooling can be a solution to this growing problem, as it can supply cooling from a heating source, with only a very small electricity input (negligible) in comparison with traditional AC systems. Due to the use of renewable energies such as solar thermal or other residua l /waste energy as heat source, the primary energy consumption is much lower than with conventional chillers.

However, solar cooling technology has a lot of barriers that delay the penetration of absorption machines in the market. The most important is the high costs of absorption chillers, as well as lack of standardisation, market familiarity and compatibility with building design methodologies. The fact that all absorption machines based on LiBr technology currently in the market need a cooling tower is also a barrier for absorption chillers market acceptance.

The objective of the CESAR project is to develop a small scale, cost-efficient absorption cooling unit, with efficient ambient air heat dissipation and which can use renewable heat sources and therefore have a minimum electricity use.

This goal will be achieved by:

- use of new working fluids and innovative fabrication aspects,
- development of efficient, compact, cost-effective and simplified heat exchangers,
- development of control systems for efficient operation of the absorption machine,
- development of a friendly-use design tool for installers and planners in order to design and dimension solar cooling systems with the new absorption machine in an easy way.

Project results:

As it is mentioned in the next section, the development of the project will conclude in five main results:

1. New knowledge on absorption technology
The main results that have been obtained concerning this point are explained in the following paragraphs. Anyway, it has to be said that new knowledge will be created within the whole project.

2. New heat exchangers for absorption cooling application
This result has been already achieved, as it is explained in the following paragraphs.

3. New control system for absorption chillers and solar cooling systems
This result has been partly reached. The algorithm has been developed but the final adjustments and validation are still required, which will be done in the following months of the project when validating the system at laboratory and real scale.

4. Design tool for solar cooling systems dimensioning
It will be achieved in the following months of the project.

5. Innovative absorption machine
It will be achieved at the end of the project. The first phase of the project has concluded in the definition of the baseline and main design specifications for the final version of the prototype, but the whole development will be completed at the end of the project with experimental validation of the machine at real scale.

Work performed so far concerning the WORKING FLUID for the absorption chiller to be developed:

The working fluid that makes possible the performance of an air-cooled absorption unit has been selected and characterised. Afterwards, chemical compatibility of this new fluid with different materials has been analysed.

After analysing the different possibilities of components addition to the H2O-LiBr working system, it has been concluded that the addition of these three salts: LiNO3, LI and LiCl seems to be the best alternative.

All the properties that are required to design the prototype with the new mixture have been measured at laboratory (the solubility, vapour pressure, thermal conductivity and viscosity). The mathematical expressions (correlations) for the measured properties have been obtained, and a library of the properties has been created to enable the use of the properties in the engineering equation solver (EES) environment, software used for the design of the components. Additionally, enthalpy has been calculated and correlated, at different compositions and temperatures, and included in the EES library as well.

Concerning the chemical compatibility of this working fluid with different materials, corrosion tests have been carried with stainless steel, copper and aluminium.

Work performed so far concerning the COMPONENTS DESIGN of the absorption chiller to be developed:

Absorber, generator, condenser and evapourator have been designed.

The absorber is the most important component of the whole system. The selected configuration for the absorber consists of a set of horizontal tubes with working solution flowing over the external tubes surface and cooling water flowing inside the tubes.

For the generator, a horizontal coil generator has been chosen as a cost-effective alternative which may be attractive for low heating load generators like the one in this project. In this configuration, the coil is immersed in a liquid pool and vapour generation takes places by nucleate boiling mechanism.

The evapourator consists on a set of horizontal tubes with water falling over the external side of the tube's bundle. The falling film of the refrigerant flows, uniformly distributed through upper tubes of the bundle using specially designed distributors. A versatile mathematical model implemented in EES was developed, which could be used for both the design and simulation of falling film evapourators for a set of geometrical configurations. The design of the evapourator was undertaken from the point of view of the optimisation of the size, trying to develop a compact, efficient, robust and cost-effective device. Enhanced tubes have been selected for the evapourator.

A falling film condenser was designed. The condenser consists on a set of horizontal tubes where water vapour is condensed over the external tubes surface. Condensed refrigerant falls in a liquid film from one tube to the next and cooling water is flowing inside those tubes. In the same way as in the case of the previously explained falling film evapourator, the design of the condenser has been carried out looking for the compactness of the device; therefore, enhanced tubes have been selected.

Potential impact:

The expected final results of the project are listed below:

1. New knowledge on absorption technology

The main final result of the project is the innovative absorption machine that will be developed. However, within the project in order to reach this objective, important knowledge on absorption technology has been created. It is mainly related to the new working fluid and new designs of the main components of the chiller. The main innovation of the project is the development of an air-cooled absorption chiller; new knowledge related to the performance analysis of the air-cooled absorption cycle with the new working fluid has been one of the main remarkable aspects. All this knowledge that has been created within the project has been transferred efficiently from the RTD performers to the SMEs of the project.

2. New heat exchangers for absorption cooling application

The design of improved heat exchangers is another important result of the project. The major innovation is that they are based on enhanced tubes that have never been used so far in absorption cooling units. These enhanced tubes improves the mass and heat transfer and makes possible a more compact and efficient chiller.

3. New control system for absorption chillers and solar cooling systems

The development of a successful control system is very important for a correct performance of a solar cooling system. The thermodynamic cycle of an absorption chiller is very sensible to the boundary conditions (outside temperature and humidity, cooling load, solar radiation, temperature in the room, etc.); therefore, a control system that at any time makes the chiller work at its optimum working conditions is important. Furthermore, besides the independent performance of the chiller, a successful control system is essential for a correct integration of the chiller in the whole cooling system. Even the absorption chiller is the best in the world, if the integration with the rest of the components (such as solar thermal collectors, storage tanks, dissipation unit, etc.) is not good, the energy efficiency of the whole system may be very low. One of the results of the CESAR project will be an optimised control system not only for the absorption chiller that will be developed, but also for the whole cooling system, which will be validated in a real building.

4. Design tool for solar cooling systems dimensioning

An easy to use design tool for installers and planners will be developed within the project. It will have a friendly interface and it will be a specific tool for the design and dimensioning of solar cooling systems with the developed absorption machine.

5. Innovative absorption machine

The development of an innovative absorption machine is the main final result of the CESAR project. After nine months of progress, the first phase in the development of the first version of the prototype has been carried out. Main specifications have been defined and the design of the main components is defined. Within the next period of the project, a prototype will be built according to design specifications, it will be validated at laboratory and the final version of the absorption chiller will be installed in a real building. The whole solar cooling system will be monitored during 4 months, from June 2013 to September 2013 in Turin (Italy).

Project website: http://www.cesarproject.net