14 Heat pumps

99103357 In-tube evaporation and condensation of R-22 and R-410A with plain and internally enhanced tubes Bogart, J. and Thors, P. Enhanced Heot Transfer, 1999, 6, (l), 37-50. Two-phase in-tube heat transfer and pressure drop comparisons for both evaporation and condensation were performed with R-22 and R-410A with a commercially available 9.53 mm internally enhanced tube-60 internal ridges with 0.2 mm (0.008 inches) ridge height and a helix angle of 18”. The experiments were performed over appropriate mass flux ranges to ensure that the data were applicable for many uses; 100-400 kg/s-m’ for the evaporation test and 200-800 kg&m’ for the condensing tests. The R-22 evaporation data from Thors and Bogart (1994) is reprinted in the article for comparison purposes and the balance of the data presented are new. The data indicate that R-410A has a substantially higher evaporation coefficient than R-22 over the bulk of the respective flow range and has a slightly higher condensation coefficient than R-22 over part of the respective flow range for a plain tube and a slightly lower condensation coefficient over the entire flow range for the enhanced tube. The results also show that R-410A has a substantially lower pressure drop than R-22 in both evaporation and condensation over the entire flow ranges.

99103358 Local flow and heat transfer of a P-row offset strip fin-tube heat exchanger Bae, J.-H. et al. Enhanced Heat Transfer, 1999, 6, (l), 13-29. A numerical study has been performed of the local flow and heat transfer characteristics of a 2-row offset strip fin-tube heat exchanger of 7 mm in tube diameter, with a view to investigating the mechanism of heat transfer in the heat exchanger. The increases in dimensionless local heat flux at the leading edge of the slit and the bottom surface of the fin has been examined quantitatively in the present study. It has been found that the air temperature downstream, the second row of the offset strip fin-tube heat exchanger, is more uniform than that of a plain plate fin-tube heat exchanger because of the augmented energy mixing effect caused by the slits. It has also found that, as the inlet air velocity is raised, the regional contribution to heat transfer decreases in the first row and increases in the second row.

99103359 A metrological analysis of a (direct digital control) DDC-based air conditioning system Capuano, A. et al. Energy and Buildings, 29, (2), 155-166. Although DDC-based air conditioning systems can potentially carry out efficiency checks, the instrumentation installed in these systems is not always suitable for making reliable plant status estimations. In order to verify the metrological performances of these systems and to define the reliability of the measurements that can be made, the authors set up a small-sized heating ventilating and air conditioning (HVAC) system, provided with standard HVAC type sensors. The measurement instrumen- tation was verified by means of calibration and field tests and the operating conditions in which it is possible to estimate in a sufficiently reliable manner the performances of each system component using the same instrumentation were experimentally determined. This analysis has shown that the sensors normally used are often highly dependent on operating and installation conditions and consequently, the efficiency estimation of each component is very unreliable when using uncalibrated sensors.

99103360 A numerical comparison of different methods for optimizing heating-restart time in intermittently occupied buildings Fraisse, G. et al. Applied Energy, 1999, 62, (3), 125-140. A brief recap of the restart time optimization methods is given: the classical methods, the methods based on fuzzy logic and the methods based on the use of a building model. In order to compare these different approaches, an intermittent heating controller, which makes it possible to define different methods for optimizing heating-restart time, has been developed as part of the TRNSYS software. Two of the six methods under study are new: the method based on fuzzy logic and the method using a two time constant building model. The numerical simulations carried out concern a building of which three rooms are studied. The changes in the various variables are presented in the case of the method based on fuzzy logic. Next, the different methods are compared according to restart precision, comfort obtained at the beginning of the occupancy period and energy consumption over the entire period of simulation.

99103361 Numerical investigations on enhancement of heat transfer in a compact fin-and-tube heat exchanger using delta winglet type vortex generators Jahromi, A. A. B. et al. Enhanced Hear Transfer, 1999, 6, (l), l-11. In this paper, three dimensional flow and heat transfer in a compact fin- tube cross flow heat exchanger was modelled as the flow in a rectangular channel with built-in cylindrical obstacles. The flow past a tube bank has been simulated by a spacewise periodic fully developed flow in a computational domain consisting of a channel clement with one tube. Flow and temperature fields in this element and in another element with two in- line tubes have been computed from the numerical solution of three dimensional Navier-Stokes and energy equations. Comparison of results show that at low Reynolds number (-400) the Nusselt number in the neighbourhood of the second tube of a two tube in-line configuration is close to the Nusselt number for spacewise periodic fully developed flow. Computational results also show poor heat transfer in the wake region. In the presence of the winglet type longitudinal vortex generators in the wake

behind the cylinder, heat transfer in this region is enhanced significantly. Results show increases in average Nusselt number due to the vortex generators can be between 20 and 50 percent in the Reynolds number range of 400 and 1200. A factor, defined as the ratio of increase in Nusselt number to increase in friction factor, of 0.65 to 0.78 can be obtained, over the range of Reynolds number mentioned above.

99103362 Optimal climate protection strategies for space heating. The case of Austria Haberl, H. et al. Energy Policy, 1998, 26, (15). 1125-1135. The debate over the costs of climate protection policies still focuses on the question of whether strategies to significantly reduce greenhouse gas emissions at zero or negative net cost (‘no regrets’ strategies) can be found. This article describes a carbon dioxide (CO*) reduction strateg for space and water heating in Austria relying on net present value analyses of 43 climate protection measures. The cost-benefit analyses include investment costs, the savings from energy conservation, the administrative costs of policy instruments and estimates of the external costs. An efficient COz reduction strategy was developed on the basis of energy supply curves which were adapted so that interactions between the CO1 reduction technologies could be considered. A cost-efficient COz reduction strategy could lower the COZ emissions for the provision of space and water heating in Austria by up to 2.7% per year relative to the official ‘business as usual’ scenario.

99103363 Study on phase change temperature distributions of composite PCMs in thermal energy storage systems Wang, J. et al. Int. J. Enemy Res., 1999, 23, (4), 277-285. Using composite phase change materials (PCMs) with different phase change temperature (PCT) distributions, a one-dimensional (1D) physical model is developed for latent heat thermal energy storage (TES) systems. By theoretical investigation under the assumption of neglecting the sensible heat, the optimum linear PCT distributions, which are corresponding to minimum phase change time, are derived. To verify the theoretical results of the optimum linear PCT distributions, the finite difference method is adopted to simulate the cyclical freezing and melting processes of composite PCMs. The numerical results in which the sensible heat is taken into account show that: (1) the optimum linear PCT distributions obtained from the theoretical analyses seem to be the optimum ones of composite PCMs in practical latent heat TES systems; (2) the phase change time of composite PCMs with the optimum linear PCT distributions used in practical latent heat TES systems can be decreased by as much as 25-40% by properly selecting the segmented numbers of composite PCMs as compared with that of PCMs of a single PCT.

99103364 Theoretical study on a novel phase change process Wang, J. et al. Int. J. Energy Res., 1999, 23, (4), 287-294. A novel homogeneous phase change process in materials (HPCP) is the focus of this study. The HPCP is analysed in detail and the expressions for one-dimensional HPCPs are derived. It is concluded that, compared with the conventional phase change processes, the complete phase change time of HPCPs can be decreased by 60% for a spherical phase change material (PCM), 50% for a cylindrical PCM and 33% for a flat plate PCM, respectively. Furthermore, the application of HPCPs to thermal energy storage systems can charge or discharge thermal energy with constant rates. Possible applications of HPCPs to thermal energy storage are simulated and further discussed using composite flat plate PCMs.

14 HEAT PUMPS 99103365 Experimental results of a sensible heat storage system for residential and light commercial application Lee, A. H. W. and Jones, J. W. Int. 1. Energy Res., 1999, 23, (4), 319-325. The performance results for a sensible heat storage system are recorded. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating and domestic water heating, this paper emphasizes the space heating performance of the unit. For different indoor and outdoor conditions to determine parameters such as heating charge rate, compres- sor power and coefficient of performance (COP) the heat storage system was tested. The thermal storage tank was able to store a full charge of heat.

99103366 Performance of a heat pump using direct expansion solar collectors Ito, S. er al. Solor Energy, 1999, 65, (3), 189-196. Theoretical and experimental studies were made on the thermal perfor- mance of a heat pump that used a bare flat-plate collector as the evaporator. The analysis used empirical equations to express the electric power consumption of the compressor and coefficient of performance (COP). as functions of temperature of evaporation at the evaporator and that of the heat transfer medium (water) at the inlet of the condenser. The experimental heat pump had a compressor with a rated capacity of 350 W and collectors with the total area of 3.24 m2. Around noon in winter the

362 Fuel and Energy Abstracts September 1999