Cooling Systems
Two types of cooling machine models are available in Polysun: Compression and heat-driven chillers. Respectively, typical cycles of such chillers are shown in following two figures. As can be seen, the main difference of the two machines is the replacement of the compressor used in the compression cycle by three main parts in the heat-driven cooling cycle meaning absorber, pump, and generator resulting in much lower electrical consumption of the later cycle compared to the other. Another advantage of a thermally-driven chiller is that a waste heat such as solar thermal energy can be used to drive the machine at moderate to high temperatures with very good matching between the solar irradiation availability and the cooling demand during summer time. However, heat-driven chiller has lower coefficient of performance than that of compression chiller.
Compression Cooling Model
The compression cooling can be provided through the current four port W/W heat pump component. The following figure shows that three operation modes can be chosen from the drop-down list.
Similar to the old heat pump model, cooling power output as well as electrical power consumption are calculated through linear interpolations of the measured points. The following figure illustrates an available template in Polysun by which heating, hot water, and cooling demand can be provided through heat pump operating in “Heating and Cooling” mode. The ground-source loop is used as low temperature heat source in heating mode and in cooling mode it is used as heat sink. The necessary pressure failure set points have been implemented for cooling mode as well.
Heat-Driven Cooling Model
The heat-driven chiller model in Polysun is based on absorption chiller cycle. Several theoretical or empirical methods have been proposed so far by researchers to model and simulate absorption chiller cycle. An analytic solution of the governing equations of the single-effect closed-cycle absorption chiller has been suggested by Kim et al. [1] which is used in Polysun. The main advantage of this model is that it can enable a quick simulation of absorption system with minimal information on working fluids and operation condition. The model is based on the heat exchanger effectiveness definition, Dühring equation and thermodynamic principles of the main constitutive components. The absorption machine model has three pairs of connecting ports enabling heat exchange between heat source, heat sink, and cooling load fluid domains over the component. The icons of absorption chiller and recooler (a heat sink to reject the heat from the absorption chiller’s condenser to the ambient through cooling water circulation) are shown in the following figure.
A number of different absorption cooling system configurations are possible to be simulated in Polysun. For an instance, a system is shown in the next figure in which solar thermal collector field as well as the auxiliary gas boiler are used to provide heating demand during winter and to run the absorption chiller as heat sources during summer to compensate for cooling demand of the building. Domestic hot water demand is also provided through such a system all over the year. The type of recooler is wet in this layout which can be replaced by dry recooler, ground-loop, or pool. Two separated heat and hot water storage tanks are used in this system layout.
Ad- and Absorption Chillers
The notion of six port absorption chiller model is depicted in the following figure. The chiller is connected to three fluid domains namely hot water circuit (left side red ports), cooling water circuit (upper side pink ports), and chilled water circuit (right side blue ports). The main varying inputs into the chiller model are inlet hot, cooling, and chilled water temperatures and flow rates. The constant parameters used in the model are absorber, condenser, evaporator, generator, and solution heat exchanger effectiveness values and also solution mass flow rate circulated inside the machine. These values are called from absorption chiller catalog. Therefore, if necessary, these constant parameters should be changed carefully in the catalog based on information obtained from the chiller manufacturers. Such a component is flexible to be connected to different kinds of heat source such as solar thermal, heat sink such as wet/dry recoolers, pool, or ground-loop, and load such as fancoil, ceiling cooling etc.
The dialog window of the absorption chiller is shown in the next figure. As can be seen, different refrigerant/absorbent pairs i.e. Water/LiBr, Ammonia/Water, Water/LiCl, Water/ CaCl2 can be defined in the catalog. Design water temperatures and flow rates are also possible to determine.
The four failure set point temperatures i.e. high and low heat sink temperature failure, high generator temperature failure, and low chilled water temperature failure have been implemented and can be set by user. These temperatures constrain the operation of the cooling machine to avoid abnormal condition e.g. crystallization phenomenon or very low performance of the chiller happens. As soon as one or more than one of these temperatures occurs during operation, machine will be switched off for a certain time span (Switch-off time) which can be defined by user. All mentioned controlling parameters are to be set according to the manufacturer technical advice.
The typical flow chart of the power exchange calculation over the absorption chiller model as well as controlling criteria on the operation condition is shown in the next figure. It is a magnificent fact that the proper controller(s) should be implemented on the chiller and the surrounding loops. The corresponding controlling strategies should be also set inside the controllers considering the cooling concept. It is always worth reading the available tool tips attributed to the properties inside the controllers.
Recoolers
As described above, different types of recoolers can be connected to the chiller component as heat sink. Figure 120 shows the dialog window of the “Wet recooler” or so called “Cooling tower” model. Cooling towers use the principle of evaporative or “wet-bulb“ cooling in order to reject the heat from water. The main advantages over a conventional heat exchanger are:
- They can achieve water temperatures below the temperature of the air used to cool it.
- They are smaller and cheaper for the same cooling load.
The main disadvantage of cooling towers is their need for careful maintenance to minimize the risk of water fouling and water-borne organisms e.g. Legionnaire’s disease.
There are two main types of cooling tower: forced draught and natural draught. However, their principles of operationare identical. The present model concerns the forced draught type.
The model is based on steady-state condition using energy balance, mass balance, and mass diffusion relations on the incremental volume. The associated differential equations are simplified by using effectiveness approach model and the Merkel’s assumptions that is neglecting the effect of the water loss due to evaporation as [2]. The following assumptions are also made:
- Heat and mass transfer in the direction normal to flows only.
- Negligible heat and mass transfer through tower walls to the environment.
- Negligible heat transfer from the tower fans to the air or water streams.
- Uniform temperature throughout the water stream at any cross section.
- Uniform cross-sectional area of the tower.
To obtain accurate results, it is important to choose/implement an appropriate cooling tower according to the absorption chiller component size. As a rule of thumb, the cooling capacity of a cooling tower is almost twice as cooling capacity of the absorption chiller connected to it. Usually, cooling towers are rated based on their design cooling capacity, design water flow rate, design air flow rate, design water inlet/outlet temperatures, and also design approach temperature. The main two performance figures of a cooling tower are approach temperature and thermal efficiency:
Approach temperature: cooling water outlet temperature- inflow air wet-bulb temperature
Thermal efficiency: (cooling water inlet temperature – cooling water outlet temperature)/(cooling water inlet temperature – inflow air wet bulb temperature)*100 (%)
It can be observed that the thermal efficiency will increase as cooling water outlet temperature approaches the inflow air wet-bulb temperature. In another word, the smaller approach temperature also implies better thermal efficiency. Typical acceptable approach temperature is above 2.
As can be seen in the next figure, the variable fan speed is also possible to apply. This feature would bring the advantage of reducing the fan power consumption during the part load condition. The operation concept of the variable fan speed is depicted in the following figure.
Validation
The validation is always an important step in the modelling. A set of measured cooling powers of an existing absorption chiller machine were obtained. Comparison between such data and the corresponding results calculated by the implemented model is shown in the next figure. The design figures of the real machine are listed in the following table:
Table: Technical parameters of the absorption chiller
| Absorption chiller type | Single-effect, Refrigerant/Absorbent: Water/LiBr |
| Design cooling power (kW) | 35.14 |
| Design COP | 0.65 |
| Design chilled water inlet/outlet temperatures (°C) | 12.5/7 |
| Design chilled water flow rate (l/hr) | 5496.4 |
| Design cooling water inlet temperature (°C) | 31 |
| Design cooling water flow rate (l/hr) | 18351.7 |
| Design hot water inlet temperature (°C) | 88 |
| Design hot water flow rate (l/hr) | 8630.7 |
The following figure reveals a good agreement between measured and calculated powers, especially in the neighbourhood of design condition, at different hot water and cooling water inlet temperatures.
