Design Optimization of Heat Wheels for Energy Recovery in HVAC Systems

Air to air heat exchangers play a crucial role in mechanical ventilation equipment, due to the potential primary energy savings both in case of refurbishment of existing buildings or in case of new ones. In particular, interest in heat wheels is increasing due to their low pressure drop and high effectiveness. In this paper a detailed optimization of design parameters of heat wheels is performed in order to maximize sensible effectiveness and to minimize pressure drop. The analysis is carried out through a one dimensional lumped parameters heat wheel model, which solves heat and mass transfer equations, and through appropriate correlations to estimate pressure drop. Simulation results have been compared with experimental data of a heat wheel tested in specific facilities, and good agreement is attained. The device optimization is performed through the variation of main design parameters, such as heat wheel length, channel base, height and thickness and for different operating conditions, namely the air face velocity and the revolution speed. It is shown that the best configurations are achieved with small channel thickness and, depending on the required sensible effectiveness, with appropriate values of wheel length and channel base and height

A trigeneration system based on polymer electrolyte fuel cell and desiccant wheel - Part B: Overall system design and energy performance analysis

This paper represents the second part of a major work focusing on a trigeneration system integrating a low temperature polymer electrolyte fuel cell (PEMFC) and a desiccant wheel-based air handling unit. Low temperature PEMFC systems have a significant potential in combined heating, cooling and power applications. However cogenerated heat temperature is relatively low (up to 65–70 C), resulting in low efficiency of the cooling process, and the fuel processor is far from being flexible, hindering the operation of the system at low load conditions. Therefore a trigeneration system based on PEMFC should be carefully designed through accurate simulation tools. In the current paper a detailed analysis of the energy performance of the trigenerative system is provided, taking into account constraints of real applications, such as PEMFC part load behavior, desiccant wheel effectiveness, heat storage losses and air handling unit electrical consumptions. The methodology adopted to model system components is deeply described. Energy simulations are performed on yearly basis with variable building air conditioning loads and climate conditions, in order to investigate the optimal trigenerative unit size. A sensitivity analysis on crucial design parameters is provided. It is shown that constrains of actual applications have relevant effects on system energy consumption, which is significantly far from expected values based on a simplified analysis. Primary energy savings can be positive in winter time if the ratio of PEMFC heating capacity to air conditioning peak heating load is close to 0.15. Instead on yearly basis primary energy savings cannot be achieved with present components performance. Positive savings can be potentially achieved if PEMFC system and auxiliary devices are properly improved

The effect of inlet velocity and unbalanced flows on optimal working conditions of silica gel desiccant wheels

AbstractThe effect of inlet velocity for optimal revolution speed and process angle of a silica gel desiccant wheel is investigated in the present work by the help of a numerical model. This is a crucial topic that involves all Variable Air Volume systems in which off-design operating conditions need optimizing. It is found that process angle mostly depends on regeneration temperature while optimal revolution speed is strongly affected by inlet velocity, for both balanced and unbalanced flows.Switching from optimal process angle to area ratio equal to one may be an interesting solution to simplify mounting of the wheel and overall air handling units arrangement. Therefore, the effect of operating the wheel with unbalanced inlet velocities and equal area fraction is then discussed

On the control of desiccant wheels in low temperature drying processes

Desiccant wheel based air handling units are of great interest in drying processes, such as in the pharmaceutical and food industries, due to the significant energy savings that can be achieved compared to conventional systems. Units based on desiccant wheels are usually optimized in peak conditions, while little attention is given to operation at part load and off design conditions. The aim of this work is to analyze the effects of different control strategies of desiccant wheels on regeneration heat consumption. The analysis is performed through a phenomenological desiccant wheel model, which is validated with experimental data collected in typical working conditions of the drying room investigated in this work. Five control strategies are proposed, highlighting that each one leads to significantly different heat consumption. Finally, an additional simplified control is introduced, showing that it can effectively reduce thermal power consumption compared to conventional control strategies