Proposal for a new hybrid control strategy of a solar desiccant evaporative cooling air handling unit

Abstract

Correctly controlled solar desiccant evaporative cooling is an interesting option for achieving savings in building air-conditioning consumption. The operation of this system (open loop cooling cycle) is strongly influenced by indoor and outdoor air conditions. This influence is characterized using numerical simulations. First the air conditioned room and the cooling system are simulated using a validated model of the desiccant wheel. Then the influence of each parameter of the desiccant air handling unit is evaluated. The third step is to assess the system cooling power for each operating mode with fluctuating outdoor and indoor air conditions. This allows for making relevant choices for a new control strategy taking into account both indoor and outdoor air conditions. This control strategy is tested for a whole cooling season and compared to a reference compression system with promising results, allowing for energy savings of about 40% for French climate.

Introduction

The continuous growth of users’ requirements toward thermal comfort, linked with the likely global warming to come is one of the main challenges of building energy research. In order to limit the energy demand for cooling applications, it is necessary to develop air-conditioning technologies in a more environment-friendly way. The fact that the highest cooling loads occur in time when high solar radiations are available – on a daily and a yearly scale – motivates the study and the development of solar air-conditioning technologies. At the present time there are three types of solar air-conditioning technologies commercially available: two closed cycles (absorption and adsorption) that are water-based cooling loop and one open cycle (solid desiccant cooling) that is air-based cooling loop, with an annual thermal coefficient of performance (COP) between 0.4 and 0.75, which is quite low [1], [2]. That is why these systems must be improved in order to, first, enhance their intrinsic performance, and second to maximize the solar part of the heat supply. These improvements will allow obtaining greater primary energy savings and will make solar cooling more profitable.

Solar desiccant evaporative cooling (SDEC) is an open cycle, which principally leans on the use of water and its potential of phase change to cool the supply air in the building. Several theoretical studies have shown its interest. Panaras et al. [3] investigated the achievable working range for these systems. Davanagere et al. [4], [5] performed a feasibility study of solar desiccant air-conditioning units, with vapor compression backup system for residential buildings in USA. Results demonstrated that the cooling requirements are met, but with a very long payback time. Mavroudaki et al. [6] looked into the potential of SDEC in southern Europe. Results showed that energy costs associated with this system increase sharply as latent gains rise.

Fig. 1, Fig. 2 show a schematic installation of the solar desiccant cooling system and the air evolution in the psychometric chart. In order to maximize the effect of the latent heat of vaporization of water, the ventilated air flow is first of all dried out in a “desiccant wheel” [A → B]. It is next cooled in a sensible heat exchanger (recovery wheel) [B → C] and at last cooled down adiabatically [C → D] through a humidifier. The operating of such a system necessitates a regeneration air flow. The air is first extracted from the building, then, after being cooled down adiabatically [E → F] in a humidifier, it cools the air of the process in the recovery wheel [F → G]. The last operation is to regenerate the desiccant material [H → I] with the return air stream that has been heated [G → H]. The regeneration heat can be taken from solar collectors and fossil back-up energy, via a storage tank. The required temperatures are in between 50 and 85 °C and therefore liquid flat plate collectors can be used.

SDEC is an open cooling cycle, which means that the refrigerant fluid is the ambient air. That is why the outdoor and indoor air conditions influence strongly its operation. The control strategy of this system is thus essential but has been little studied in the literature. Five operating modes were listed by Henning [7], namely free cooling (usual ventilation), indirect humidification (recovery wheel and return humidifier switched on), combined humidification (recovery wheel, return and process humidifiers switched on) desiccant mode, and desiccant mode with increased airflows. In [8], the controlled parameters were the recovery wheel rotation speed, which modifies its efficiency, the regeneration temperature and the ventilation flow. The measured values (outdoor temperature and the difference between indoor set point temperature and indoor temperature) were used to calculate a cooling demand function. The supply air temperature was monitored as well to determine the shift of the operating mode. Ginestet et al. [9] investigated the use of indoor temperature and the differential between indoor and supply temperature as measured values. Controlled parameters were the regeneration temperature and the ventilation flow rates. The authors noticed that night cooling is essential in order to ensure the control of the temperature rise. One important point is that usually, the control strategy is used and designed to first ensure indoor thermal comfort. As mentioned before, indoor and outdoor air conditions (temperature and humidity) influence a lot the desiccant air handling unit cooling power. The control strategy must indeed allow using the AHU with the proper operating mode, the one with the best efficiency and with a sufficient cooling power required by the cooling demand.

Consequently, the methodology proposed here to elaborate energy efficient control strategy is as follows:

• Modeling and validation of the solar desiccant cooling system.

• Evaluation of parameters’ influence for each operating mode of the desiccant AHU. The aim is to identify relevant controlled parameters for the system.

• Evaluation of the cooling power of each operating mode for several indoor air temperatures and outdoor air temperatures and humidity ratios. This way, a map of the desiccant plant performance is developed; helping to elaborate the operating mode decision scheme.

• Proposition of a new control strategy, based on previous steps.

• Evaluation of the performance of the new control strategy. Tests on two typical days and the energy consumption for the whole cooling season are presented.