ALBERT

Description: A general design procedure is presented for solar-assisted series heat pump systems used for space and process water heating. The procedure accounts for the variable efficiency and rate of energy delivery by the heat pump. The fraction of the required energy supplied by solar and the fractions supplied by work and auxiliary are determined. The performance results from this design procedure are compared against detailed computer simulations on a monthly and seasonal basis. For low temperature space heating applications, the maximum difference between the design procedure and computer simulations is three percent on a seasonal basis and 12 percent on a monthly basis. For high temperature process water heating systems, the maximum differences are three to six percent on a seasonal basis and six to 12 percent on a monthly basis, with the accuracy dependent upon the control strategy used. This design procedure is used to investigate high temperature series heat pump systems for providing hot water for industrial processes. Various system parameters are evaluated in terms of the overall effect that each has on system performance and design. Recommendations are given for high temperature series heat pump system design.

Description: A comparative study of the thermal and economic performance of the parallel and series solar-heat pump systems, stand-alone solar, and stand-alone heat pump systems for residential space and domestic hot water heating has been undertaken for the United States using FCHART 4.0 [1]. The results are useful for a regional assessment of the viability of the different systems, and for assessing policies that will encourage the implementation of the most energy efficient system. The magnitude of the potential energy savings was determined for each system on the basis of an equal total system cost in the case of the series, parallel, and solar systems. The cost was governed by current federal tax credits, and found to be 10,000 dollars. The size and cost of the heat pump are the same in the series, parallel, and stand-alone heat pump systems. A line can be drawn across the United States north of which the parallel heat pump system saves the most energy, and south of which the solar system saves the most. The better of either the solar or the parallel systems consistently used less energy than either stand-alone heat pump or series systems for all locations. The conventional oil or gas furnace seasonal efficiency which would be required to save as much primary energy as the better alternative system was identified regionally. In all but the northern portions of the United States, conventional furnaces would use more primary energy than the better alternative system. The price that the solar collector in the series heat pump system would have to be so that a larger collection system could be installed and the series system would match the energy savings of the preferred system, whether solar or parallel heat pump, was calculated. This price was one-half to two-thirds of current collector prices. The break-even electricity price was determined which is the price below which the life cycle savings of the alternative system are positive. The better alternative was found to be economic against oil furnaces in all regions of the U.S., but economic against gas furnaces only in the Southwest.

Description: This paper presents a second law analysis of solid desiccant rotary dehumidifiers. The equations for entropy generation for adiabatic flow of humid air over a solid desiccant are developed. The generation of entropy during operation of a rotary dehumidifier with infinite transfer coefficients is investigated and the various sources of irreversibility are identified and quantified. As they pass through the dehumidifier, both the process and regeneration air streams acquire nonuniform outlet states, and mixing both of these air streams to deliver homogeneous outlet streams is irreversible. Transfer of mass and energy between the regeneration air stream and the desiccant matrix occurs across finite differences in vapor pressure and temperature and these transfer processes generate entropy. The second law efficiency of the dehumidifier is given as a function of operating conditions and the effect of finite transfer coefficients for an actual dehumidifier is discussed. It is shown that operating the rotary dehumidifier at conditions that minimize regeneration energy also yields a local maximum for the second law efficiency.

Description: Previous investigators have shown that an internally reversible Carnot cycle, operating with heat transfer limitations between the heat source and heat sink at temperatures TH and TL, achieves maximum power at an efficiency equal to 1−TL/TH independent of the heat exchanger transfer coefficients. In this paper, optimization of the power output of an internally irreversible heat engine is considered for finite capacitance rates of the external fluid streams. The method of Lagrange multipliers is used to solve for working fluid temperatures which yield maximum power. Analytical expressions for the maximum power and the cycle efficiency at maximum power are obtained. The effects of irreversibility and economics on the performance of a heat engine are investigated. A relationship between the maximum power point and economically optimum design is identified. It is demonstrated that, with certain reasonable economic assumptions, the maximum power point of a heat engine corresponds to a point of minimum life-cycle costs.

Description: Many previously studied natural convection enclosure problems in the literature have the bounding walls of the enclosure responsible for driving the flow. A number of relevant applications contain sources within the enclosure which drive the fluid flow and heat transfer. The motivation for this work is found in solar thermal storage tanks with immersed coil heat exchangers. The heat exchangers provide a means to charge and discharge the thermal energy in the tank. The enclosure is cylindrical and well insulated. Initially the interior fluid is isothermal and quiescent. At time zero, a step change in the source temperature begins to influence the flow. The final condition is a quiescent isothermal fluid field at the source temperature. The governing time-dependent Navier-Stokes and energy equations for this configuration are solved by a finite element method. Solutions are obtained for 103≤RaD≤106. Scale analysis is used to obtain time duration estimates of three distinct heat transfer regimes. The transient heat transfer during these regimes are compared with limiting cases. Correlations are presented for the three regimes.

Description: The performance of open-cycle desiccant air conditioners for residential applications is evaluated. The performance of these systems is compared to that of vapor compression air conditioners on the basis of primary energy use and cost. Systems with improved dehumidifiers can achieve seasonal COP’s on the order of 1.1. These systems, when coupled with a solar energy system to supply regeneration energy, are significantly better than conventional air conditioners on a primary energy basis, but are not presently cost-competitive.

Description: The heating system for CSU House I is simulated using TRNSYS, and simulation results are compared to the measured performance. The heating system is composed of a liquid collection and storage system, a domestic hot water system, and an air delivery system. The components were modeled using standard TRNSYS components. Measured weather data from the site were employed as the driving function. Measured energy quantities, including the useful gain from solar, that supplied to the house and the domestic hot water system, and the auxiliary were compared to those from the simulation for three periods of six to eleven days each. Comparisons were made on both a daily basis and over the entire period. The simulated energy quantities are found to agree with the data within the accuracy of the measurements. Simulated hourly values of storage tank temperatures are compared to those measured. Although there are differences of up to 5°C between the two at times, the agreement is generally within 2°C. These results help establish the validity of simulation methods for system analysis. Conventional engineering techniques can be used to formulate component models and determine parameter values. Verified simulation models can be then used to predict long term system performance.

Description: A general procedure is presented for estimating the seasonal performance of solar-assisted heat pumps with refrigerant-filled collectors. The procedure accounts for variations in collector design and orientation and also for heat pump capacity and efficiency. The results from this design procedure for space heating applications are compared against those for both conventional heat pumps and liquid solar systems. The effects of collector area and design, heat pump size and degradation due to cycling, and energy storage are discussed. For space heating, uncovered collector heat pump systems have better performance than both conventional air-source heat pumps and covered collector heat pump systems over a wide range of collector areas. The cooling performance of a collector heat pump system is inferior to that of conventional heat pumps.

Description: A heuristic “pseudo-steady-state” model of the heat and mass transfer occurring in desiccant dehumidifiers, embodied in the program DESSIM, has been proposed as a conceptually and numerically simple analysis tool (Barlow, 1982). A comparison is made with a finite difference solution to determine the accuracy and limitations of the pseudo-steady-state model. The comparison indicates that the pseudo-steady-state model can produce accurate results of dehumidifier performance relative to the finite difference solution when used carefully, although at greater computational expense. Substitution of the finite difference solution into the overall DESSIM program results in a potentially accurate and useful analysis tool.