Experimental evaluation of a single-stage heat transformer operating with the water/Carrol™ mixture
Introduction
Energy is an essential element for economic development and social progress in all countries. Without adequate and secure supplies of energy, objectives of economic and social development are unlikely to be met. Currently, crude oil, natural gas (NG) and coal account for about 90% of the world's commercial energy production. The rest is provided by a variety of sources which include nuclear power, geothermal energy and renewable resources.
The primary world energy demand is estimated to be 412.2 EJ/yr (1 EJ=1018 J). The proven world oil reserves (as of 1 January 1995) are estimated to be 1005 billion barrels (6301.35 EJ thermal equivalent). The proven world gas reserves are estimated to be 135,000 billion m3 (5251.5 EJ thermal equivalent) and the proven world coal reserves are 528 billion mt (14,731.2 EJ thermal equivalent) [1]. World oil, NG and solid fuel demands are expected to rise at annual averages of 1.8, 3.0 and 2.1%, respectively, during the period 1991–2010. Demands are expected to grow at double these rates in developing countries [2]. Based on this information, it is clear that in the foreseeable future there will be serious problems with energy supplies. For these reasons, as well as environmental concerns, many countries are trying to reduce their energy consumption. Some countries are investing considerable amounts of money in the development of equipment that facilitates the recovery and efficient use of energy.
Some of the most interesting devices for energy savings, which consume negligible amounts of primary energy, are heat-driven absorption heat-pump systems. These and especially absorption heat transformers are some of the most promising devices for upgrading industrial waste heat and heat from geothermal and solar sources to higher temperature levels.
Absorption heat transformers are devices for increasing the temperatures of moderately warm sources to more useful levels. Typically, up to half of the heat supply is increased in temperature while the rest is discharged at a lower temperature.
There are currently about 15 heat transformers operating in industrial plants worldwide 3, 4, 5. Although the water/lithium–bromide mixture has been the only mixture used commercially in the heat transformers, its use has the following major disadvantages: (i) low working pressures, (ii) high corrosivity at temperatures above 150°C, (iii) propensity to crystallization at high LiBr concentrations, and (iv) limited gross temperature lift (GTL) at values of about 45°C. For these reasons new working pairs have been proposed and used in absorption heat transformers 6, 7, 8, 9, 10, 11. A system which reduces some of the main disadvantages of the water/lithium–bromide mixture is the water/Carrol™ mixture. In this paper, we present an experimental evaluation of an absorption heat transformer operating with this system.
Section snippets
Thermodynamic cycle
A single-stage heat transformer (SSHT) consists of an evaporator, a condenser, generator, an absorber, and economizer. Fig. 1 shows a SSHT in a plot of temperature against pressure. A quantity of waste heat QGE is added at a relatively low temperature TGE to the generator to vaporize the working fluid from the weak salt solution containing a low concentration of absorbent. The vaporized working fluid flows to the condenser delivering an amount of heat QCO at a reduced temperature TCO. The
Experimental studies
The experimental single-stage heat transformer (SSHT) operated at the IIE, Mexico, was designed and constructed at Salford University in the UK [12]. It was subsequently donated to the IIE, where it was modified and operated with different working mixtures [13]. The equipment consisted of standard supplies from Quickfit Limited, UK, which could be easily replaced. Fig. 2 shows a diagram of the system which shows the locations of the main components such as the generator, absorber, condenser,
Instrumentation
Experimental evaluation of the SSHT is based on data derived from tests. Temperature, pressure, mass-flow rate, concentration, and electrical power were measured. We used type E thermocouples with a maximum error of 1.7% for temperatures greater than 0°C. To facilitate heat transfer, thermocouple pockets were filled with vacuum oil. The location of the thermocouples is shown in Fig. 2. Two transducers were utilized to measure the pressure differences between the atmosphere and absorber and
Thermodynamic considerations
For the process show in Fig. 1 and from heat and mass balances for the main components, the following equations were obtained for calculations of the FR, GTL, solution heat-exchanger effectiveness, internal and external heats in the main components, heat lost, and COPs for the heat transformer. Since the absorbent does not evaporate in the temperature range under consideration (XWF=0), the FR for this system is
The GTL for the system is
The external heats
Experimental results
Fig. 3 shows the variation of the GTL vs the concentration of the strong salt solution. It is seen that the GTL increases with an increase in the solution concentration and reaches 52°C for the greatest concentration. Fig. 4 is a plot of the temperature lift vs the evaporator temperature for different strong solution concentrations. We see that it increases with an increase in the evaporator temperature and with the concentration of the strong solution. Fig. 5Fig. 6 are plots of the external
Conclusions
The GTL with a single-stage glass heat transformer increases with the salt solution concentration and with the absorber and evaporator temperatures. The COP decreases with both the GTL and absorber temperature but increases with the evaporator and generator temperatures. The COPs were in the range 0.1–0.2 for the mixture. These values are lower than expected because the high heat losses varied between 20 and 35% of the heat supplied. The highest GTL was 52°C, which is greater than values
References (16)
- et al.
Int J Refrig
(1991) - et al.
Heat Recovery Systems and CHP
(1994) - et al.
Heat Recovery Systems and CHP
(1995) - et al.
Heat Recovery Systems and CHP
(1992) - Shell Briefing Service. Energy in Profile...
- International Energy Agency, OECD. World Energy Outlook...
- Mashimo K. In: Proceedings of the IEA Heat Pump Conference, Michigan (USA),...
- Jansen PF, Wormgoor JW. In: Proceedings of the 3rd Int. Symp. on the Large Scale Applications of Heat Pumps, Oxford...