Solar thermal collectors and applications

Absorption units

Absorption is the process of attracting and holding moisture by substances called desiccants. Desiccants are sorbents, i. e. materials that have an ability to attract and hold other gases or liquids, which have a particular affinity for water. During absorption the desiccant undergoes a chemical change as it takes on moisture; for example, the table salt, which changes from a solid to a liquid as it absorbs moisture. The characteristic of the binding of desiccants to moisture, makes the desiccants very useful in chemical separation processes [143].

Absorption systems are similar to vapour-compression air conditioning systems but differ in the pressurisation stage. In general an absorbent, on the low-pressure side, absorbs an evaporating refrigerant. The most usual combi­nations of fluids include lithium bromide-water (LiBr-H2O) where water vapour is the refrigerant and ammonia-water (NH3-H2O) systems where ammonia is the refrigerant.

The pressurisation is achieved by dissolving the refrigerant in the absorbent, in the absorber section (Fig. 37). Subsequently, the solution is pumped to a high pressure with an ordinary liquid pump. The addition of heat in the generator is used to separate the low-boiling refrigerant from the solution. In this way the refrigerant vapour is compressed without the need of large amounts of mechanical energy that the vapour-compression air con­ditioning systems demand.

The remainder of the system consists of a condenser, expansion valve and evaporator, which function in a similar way as in a vapour-compression air conditioning system.

The NH3-H2O system is more complicated than the LiBr-H2O system, since it needs a rectifying column that assures that no water vapour enters the evaporator where it could freeze. The NH3-H2O system requires generator temperatures in the range of 125-170 °C with air-cooled absorber and condenser and 95-120 °C when water-cooling is used. These temperatures cannot be obtained with FPCs. The coefficient of performance (COP), which is defined as the ratio of the cooling effect to the heat input, is between

0. 6 and 0.7.

The LiBr-H2O system operates at a generator tempera­ture in the range of 70-95 °C with water used as a coolant in the absorber and condenser and has COP higher than the NH3-H2O systems. The COP of this system is between 0.6 and 0.8 [97]. A disadvantage of the LiBr-H2O systems is that their evaporator cannot operate at temperatures much below 5 °C since the refrigerant is water vapour. Commer­cially available absorption chillers for air conditioning applications usually operate with a solution of lithium bromide in water and use steam or hot water as the heat source. In the market two types of chillers are available, the single and the double effect.

The single effect absorption chiller is mainly used for building cooling loads, where chilled water is required at 6-7 °C. The COP will vary to a small extent with the heat source and the cooling water temperatures. Single effect chillers can operate with hot water temperature ranging from about 80 to 150 °C when water is pressurised [118].

The double effect absorption chiller has two stages of generation to separate the refrigerant from the absorbent. Thus the temperature of the heat source needed to drive the high-stage generator is essentially higher than that needed for the single-effect machine and is in the range of 155-205 °C. Double effect chillers have a higher COP of about 0.9-1.2 [144]. Although double effect chillers are more efficient than the single-effect machines they are obviously more expensive to purchase. However, every individual application must be considered on its merits since the resulting savings in capital cost of the single-effect units can largely offset the extra capital cost of the double effect chiller.

The Carrier Corporation pioneered lithium - bromide absorption chiller technology in the United States, with early single-effect machines introduced around 1945. Due to the success of the product soon other companies joined the production. The absorption business thrived until 1975. Then the generally held belief that natural gas supplies were lessening, let to US government regulations prohibiting the use of gas in new constructions and together with the low cost of electricity led to the declination of the absorption refrigeration market [145]. Today the major factor on the decision on the type of system to install for a particular application is the economic trade-off between the different cooling technologies. Absorption chillers typically cost less to operate, but they cost more to purchase than vapour compression units. The payback period depends strongly on the relative cost of fuel and electricity assuming that the operating cost for the needed heat is less than the operating cost for electricity.

The technology was exported to Japan from the US early in the 1960s, and the Japanese manufacturers set a research and development program to improve further the absorption systems. The program led to the introduction of the direct - fired double-effect machines with improved thermal performance.

Today gas-fired absorption chillers deliver 50% of commercial space cooling load worldwide, but less than 5% in the US, where electricity-driven vapour compression machines carry the majority of the load [145].

Many researchers have developed solar assisted absorp­tion refrigeration systems. Most of them have been produced as experimental units and computer codes were written to simulate the systems. Some of these designs are presented here.

Hammad and Audi [146] described the performance of a non-storage, continuous, solar operated absorption refriger­ation cycle. The maximum ideal COP of the system was determined to be equal to 1.6, while the peak actual COP was determined to be equal to 0.55.

Haim et al. [147] performed a simulation and analysis of two open-cycle absorption systems. Both systems comprise a closed absorber and evaporator as in conventional single stage chillers. The open part of the cycle is the regenerator, used to reconcentrate the absorber solution by means of solar energy. The analysis was performed with a computer code developed for modular simulation of absorption systems under varying cycle configurations (open - and closed-cycle systems) and with different working fluids. Based on the specified design features, the code calculates the operating parameters in each system. Results indicate a definite performance advantage of the direct-regeneration system over the indirect one.

Hawlader et al. [148] developed a lithium bromide absorption cooling system employing an 11 X 11m2 collector/regenerator unit. They also have developed a computer model, which they validated against real exper­imental values with good agreement. The experimental results showed a regeneration efficiency varying between 38 and 67% and the corresponding cooling capacities ranged from 31 to 72 kW.

Ameel et al. [149] give performance predictions of alternative low-cost absorbents for open cycle absorption using a number of absorbents. The most promising of the absorbents considered, was a mixture of two elements, lithium chloride and zinc chloride. The estimated capacities per unit absorber area were 50-70% less than those of lithium bromide systems.

Ghaddar et al. [150] presented modelling and simulation of a solar absorption system for Beirut. The results showed that, for each ton of refrigeration, it is required to have a minimum collector area of 23.3 m2 with an optimum water storage capacity ranging from 1000 to 15001, for the system to operate solely on solar energy for about 7 h per day. The monthly solar fraction of total energy use in cooling is determined as a function of solar collector area and storage tank capacity. The economic analysis performed showed that the solar cooling system is marginally competitive only when it is combined with domestic water heating.

Erhard and Hahne [151] simulated and tested a solar- powered absorption cooling machine. The main part of the device is an absorber/desorber unit, which is mounted inside a concentrating solar collector. Results obtained from field tests are discussed and compared with the results obtained from a simulation program developed for this purpose.

Hammad and Zurigat [152] described the performance of a 1.5 ton solar cooling unit. The unit comprises a 14 m2 flat - plate solar collector system and five shell and tube heat exchangers. The unit was tested in April and May in Jordan. The maximum value obtained for actual COP was 0.85.

Zinian and Ning [153] describe a solar absorption air conditioning system which uses an array of 2160 evacuated tubular collectors of total aperture area of 540 m2 and a LiBr absorption chiller. Thermal efficiencies of the collector array are 40% for space cooling, 35% for space heating and 50% for domestic water heating. It was found that the cooling efficiency of the entire system is around 20%.

A new family of ICPC designs was developed by Winston et al. [154] which allows a simple manufacturing approach to be used and solves many of the operational problems of previous ICPC designs. A low concentration ratio is used that requires no tracking together with an off - the-shelf 20 ton double effect LiBr direct fired absorption chiller, modified to work with hot water. The new ICPC design and double effect chiller was able to produce cooling energy for the building using a collector field that was about half the size of that required for a more conventional collector and chiller.

A method to design, construct and evaluate the performance of a single stage lithium bromide-water absorption machine is presented in Ref. [155]. In this the necessary heat and mass transfer relations and appropriate equations describing the properties of the working fluids are specified. Information on designing the heat exchangers of the LiBr-water absorption unit is also presented. Single­pass vertical-tube heat exchangers have been used for the absorber and for the evaporator. The solution heat exchanger was designed as a single-pass annulus heat exchanger. The condenser and the generator were designed using horizontal tube heat exchangers.