The ECOS performance assessment: computer simulations

Mathematical modelling activities which focused on the creation of a simplified algorithm capable to represent the heat and mass transfer processes within the ECOS have been carried out [3]. The dynamic model, describes the transient heat and mass transfer processes within the air conditioner. The algorithm was implemented aiming to create a software tool capable to carry out a large number of simulations in an acceptable time. It allows the assessment of the energetic performance at different supply air conditions (i. e., temperature and humidity) and thereby to simulate a whole process consisting of the three phases, namely adsorptive air-conditioning, regeneration and pre-cooling.

It was assumed that two heat exchangers are periodically operated. While one heat exchanger dehumidifies and cools the outside air, the other one is regenerated with hot air and then pre-cooled with outside air. The duration of the regeneration and pre-cooling phases have been assumed 80% and 20% of the adsorptive phase length (from here on referred as duration of the cycle), respectively. Moreover it has been assumed that a heat recovery component operates between exhaust air and outside air during the regeneration, resulting in a pre-heating of the regeneration air stream. The heat recovery efficiency, considered constant, was set to 0.8. The simulation’s aim was to assess the process performance in hot and humid climates, therefore the following ambient air conditions were assumed: temperature 35°C, humidity ratio 20 g/kg. The return air conditions used (typical for an office building) are: air temperature 26°C and relative humidity 50%.

Using the above mentioned dynamic model a set of simulation run was performed, with a time-step of 0.01 sec. A preconditioning run (i. e., iterative simulation of an initial time period until temperatures and/or fluxes stabilize at initial values) of all the three phases was carried out before each actual calculation.

The assessment of the ECOS performance has been accomplished in terms of: supply air temperature, supply air humidity ratio and coefficient of performance (COP). The latter is calculated as follows:

Cop = Qs"i’ p|y

Qreg

Qsuppiy denotes the air-conditioning work and it is assessed as:

t=t*

Qs"pply ^msupply ’ (hamb hs"pply) [kJ]

t=0

where hamb and hsupply are the specific air enthalpy at ambient and supply air conditions during each time step t, respectively. The limits for the integration range from zero (i. e., beginning of the cycle) to t* (i. e., cycle duration). The supply air mass flow rate is expressed as m supply.

Qreg, is the amount of energy used for the sorptive material regeneration during a cycle, and it is expressed as follows:

t=0.8-t*

Qreg “ Jmreg ’ (hrec _ hreg) [kJ]

t=0

where hrec is the specific air enthalpy at the exit of the heat recovery device operated between the ambient and the exhaust air streams. hreg is assessed using the same humidity ratio of ambient air and the regeneration temperature set for the given simulation. Once structural heat exchanger characteristics (i. e., plates material and dimensions, flow characteristics, sorptive material and its coupling with the plates, see table 1) have been fixed, the system performance is a function of the way the system is operated. In particular the cycle duration
(interval of the periodic operation) and the regeneration temperature are strongly influencing the ECOS energetic behaviour. In order to analyse the system performance a parametric study through computer simulations has been carried out.

The study has been worked out varying the regeneration temperature in a range of temperatures consistent with the aim of using solar energy as heat source (i. e., Treg 60-95°C). The cycle’s duration (t*) ranges between 150 and 600 seconds. A summary of the above mentioned data is given in table 2. In the ECOS process adsorption phase the values of the supply air temperature and humidity ratio vary within the same cycle. The variation is caused by the non-continuity of the process since the two variables change according to the rate of water vapour load of the sorbent material. Therefore, for each cycle an average value of the supply air temperature and humidity ratio has been calculated and used during the performance assessment process.