M. Sokolov

Solar-powered compression-enhanced ejector air conditioner

Abstract This article is an extension of an earlier investigation into the possibility of adaptation of the ejector refrigeration cycle to solar air-conditioning. In a previous work [1] the ejector cycle has been proven a viable option only for a limited number of cases. These include systems with combined (heating, cooling, and hot water supply) loads where means for obtaining low condensing temperature are available. The purpose of this work is to extend the applicability of such systems by enhancing their efficiency and thereby improving their economical attractiveness. This is done by introducing the compression enhanced ejector system in which mechanical (rather than thermal) energy is used to boost the pressure of the secondary stream into the ejector. Such a boost improves the performance of the whole system. Similar to the conventional ejector, the compression-enhanced ejector system utilizes practically the same hardware for solar heating during the winter and for solar cooling during the summer. Thus, it is capable of providing a year-round space air-conditioning. Optimization of the best combination in which the solar and refrigeration systems combine through the vapor generator working temperature is also presented.

Analysis of an integral compact solar water heater

The thermal performance of an integral compact solar water heater is analysed numerically and compared with experimental data. In the proposed system the collector and the water tank are built into one unit with water flow caused by thermosyphonic force. Two geometries of the proposed system are investigated. While the proposed system presents a new concept for solar water heater, the efficiency of such systems are shown to be larger than conventional ones. The numerical scheme used does not make any a priori assumption concerning temperature profiles.

Effects of connecting pipes in thermosyphonic solar systems

Abstract The influence of thermal insulation of pipes in thermosyphonic systems is simulated. In spite of their normally small heat transfer area, these pipes are capable of triggering a reverse flow which causes a substantial drop in the overall efficiency. A clear recommendation of this paper is that at least the upper pipe should be properly insulated. The systems efficiency dependence on the high difference between the collectors top and the tanks bottom is studied. The results indicate that this height should be in the range of 30–80 cm.

Experiments with an integral compact solar water heater

Abstract Experimental results are presented for an Integral Compact Solar Water Heater, which was described and numerically simulated in [1]. Temperature fields, both in the collector channel and water tank, were monitored. An attempt was made to experimentally measure water mean velocity in the collector channel. The results show agreement with numerical simulations and they reveal the high efficiency of the system. Increased losses during no radiation periods require that the system should be equipped with means to prevent reverse flow. Two hot water draw experiments have been conducted and the experimental results are in agreement with theoretical predictions.

Performance simulation of solar collectors made of concrete with embedded conduit lattice

A solar collector made of a lattice of fluid conduits embedded within a thin concrete slab is investigated. Such a configuration can be constructed to withstand some mechanical strain by reinforcing the concrete with glass fibers. This collector can be integrated within construction elements of buildings and therefore offers means for low-cost solar energy collection. The geometry of such a collector as well as its characteristic parameters are different from the conventional flat-plate thin-fin collector. Its performance cannot therefore be accurately predicted by assuming a thin-fin behavior. It requires a different and somewhat more involved thermal analysis. In the present analysis, a numerical solution of a two-dimensional cross-sectional slice is expanded in the longitudinal direction by superpositioning such slices in tandem. A parametric study of the relative influence of various operational, geometrical and material parameters is presented. The study provides the tools for a feasibility study of such collectors. Transient characteristics of the collectors dynamic response during a typical summer day with continuous or intermittent radiation are also presented.

Freshwater floating-collector-type solar pond

Abstract A new type of solar pond is introduced in an effort to provide an inexpensive, renewable heat source, mainly for space-heating purposes. The suggested freshwater solar pond is covered with floating collectors made of insulation boards and thin plastic sheets, and requires very little, if any, constructional work. Forced flow in the collector layer of the pond provides the reduction of the mean collector and pond temperatures and ensures high efficiency. The collectors also provide the necessary thermal and evaporation insulation so that freshwater may be used and the cumbersome and delicate task of salt-gradient maintenance is eliminated. Both simulation and experimental results are reported. A one-dimensional quasi-steady-state simple model is the basis for the simulation. The presented experimental data validate the simulations. Longterm performance characteristics as well as possible modes of seasonal storage utilization of such solar ponds are discussed.

Optimal control of a multicomponent solar collector system

Abstract An analysis of the optimal flow rate for a solar energy system with a mix of different collector brands connected in series is performed. Both a recirculating system, in which energy storage is utilized, and a once-through system, in which the working fluid does not return to the collector field, are investigated. For both cases the optimal flow rates are derived, a numerical analysis is performed, and the sensitivity of the system to the flow rate is investigated. The effect of different load profiles is shown for the recirculating system.

Thermal and optical analysis of an evacuated circular cylindrical concentrating collector

Abstract An evacuated concentrating circular cylindrical collector has been numerically investigated by ray tracing analyses. The optical efficiency of the collector is found by following incident rays onto the collector cover, calculating the amount of energy absorbed by the receiver for each ray, and then integrating the energy for all rays. Absorption and reflection losses in the collector materials are considered, as well as the formation of ray cascades. A thermal radiation exchange factor between the collector receiver and the cover, needed to find the thermal radiation losses, is also determined using ray tracing techniques. The collector overall efficiency was found for the case of a selective surface coating on the inner receiver cylinder and for the case of an absorbing fluid contained within a semi-transparent inner cylinder. The addition of a highly reflective thermal radiation coating of the inner surface of the cover, in order to suppress thermal radiation losses, was also evaluated.

Numerical evaluation of radiation absorption by a fluid confined in a semi-transparent circular cylinder

Abstract The energy absorbed by a fluid confined within a circular-cylindrical cover is calculated. Energy losses due to reflection on both sides of the inner and outer surfaces of the cover as well as the energy absorption of the cover are considered. Formation of ray cascades are taken into account. A ray tracing computer simulation was performed for various cover thicknesses, coefficients of extinction, cover diameters, and concentrations. A comparison of the energy absorbed for the circular geometry and of a flat surface geometry was also done. Optimization of the fluid diameter with respect to the amount of energy absorbed by the fluid per unit aperture was obtained.

Improving load matching characteristics of a thermosyphonic solar system by thermostatically controlled circulation

Abstract The load matching characteristics of a thermosyphonic solar water heater can be improved by utilizing a thermostatic flow control (TFC). Simulation of the performance of a typical thermosyphonic domestic solar heater, with and without a TFC, was used to evaluate the yearly requirement of auxiliary energy to meet four different loads. The amount of yearly auxiliary energy required to fully match the load demands is used as a measure of the matching improvement. Results indicate that, for load temperatures of 40°, 60°, and 70°C, the thermostatic flow controller improves the systems (multi- or single-pass) performance, while the common single-pass system without the thermostatic flow controller is the best choice for a 50°C load temperature.