Condensing Dissipators

Cooling demand in buildings is directly related to ambient conditions, par-ticularly to ambient temperature and relative humidity. These two variables directly influence the calculation of the condensing capacity in cooling towers since they influence the air’s enthalpy that enters the refrigeration system.

In the case of dry coolers, only the ambient temperature influences their condensing capacity. Ergo, cooling demand and condensing capacity are in-versely related. This might lead to a problem when generating cooling energy at the power plant.

In the hybrid polygeneration power plant configuration that has been proposed as a problem to solve in this thesis, both cooling generators are connected to the same condensing node and thus the same heat sink. It is assumed that the water temperature from the cooling generators entering and exiting the condensing dissipators are equal and fixed. The water tem-perature that is exiting the cooling tower regulates the fans that provide the air stream through the tower.

For some environmental conditions, the cooling demand leads to condens-ing power that is larger than the maximum defined for the condenser in the Equation 4.2.

When this happens, the cooling demand is not satisfied, and the user’s needs are not meet.

P_cn = ˙G_s(H₂−H₁) (4.2) whereG_s is the maximum air flow that can be forced through the tower or dry cooler and is defined by the rated power of the fans and design pa-rameters such as height, and air filters. H₁ and H₂ are the entering and exiting enthalpies of the air in the cooler. To ensure heat transfer along the cooling tower, the exiting bulk liquid temperature must have a temperature difference that is greater than 2.8^◦C with an entering air wet bulb tempera-ture according to [114]. Otherwise, the exiting bulk liquid temperatempera-ture will be of higher temperature than desired. In a cooling tower, there is an effi-ciency drop because the exiting bulk liquid temperature is seldom more than 0.3K above the exiting air stream temperature, which can be assumed to be saturated for calculation purposes [115].

The temperature that enters the tower is 33^◦C and the return should be 29^◦C. If the return temperature is higher than the 29^◦C that is fixed as the set point, the efficiency of the chiller will drop. On the other hand, the return

temperature cannot be lower than 20^◦C to avoid crystallisation problems on the absorption solution. The influence in cooling capacity of cooling tower’s return temperature to chiller is depicted in Figure 4.6.

1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2 3 4 3 6

5 0 7 5 1 0 0 1 2 5 1 5 0

Cooling Capacity (%)

C o o l i n g T o w e r T r ( º C )

Figure 4.6: Influence on cooling capacity of condensing return temperature

4.1.3.1 Cooling tower

The cooling tower is designed to dissipate the condensing energy requirements from the absorption chillers and the electric chiller. The cooling tower is open, which means that the condenser water is sprayed counter-flow with the forced air from the ventilators at the tower; the water is recovered in a tray at the bottom of the tower. The cooling tower comprises three modules of fans that each have an electrical power of 16.5kW_e and a total electrical power of 49.5kW_e. The mass flow of air (G_s) that can move through the tower is 315kg/s. The refrigeration occurs due to an exchange of enthalpy

between the condenser water and the surrounding air, which is carried out through an increase of the temperature and humidity in the incoming air. It is generally accepted that the relative humidity of the air exiting from the cooling tower is equal to 100%.

4.1.3.2 Dry cooler

The dry cooler is designed to dissipate the condensing energy requirements from the electric chiller. The refrigeration occurs due to an exchange of en-thalpy between the condenser water and the surrounding air. This exchange is carried out only through an increase of temperature in the incoming air since the relative humidity along the dry cooler does not vary. The dry cooler comprises six fans that each have an electrical power of 7.5kWe, or 45kWe

in total. The mass flow of air that can be moved through the dry cooler is 118kg/s.

4.1.4 Storage

The power plant at Parc Bit also includes energy storage. This energy is stored as hot water for heating and cold water for cooling. All the energy generators inject their energy productions into the tanks and from there the load is flown to the customer. Therefore, the tanks are also energy buffers.

Table 4.1 states that there is 200m³ of water for heating and cooling. This volume of water is separated into two water tanks. Consequently, there are four identical tanks of 100m³ each that are installed at the generation site.

The tanks are cylindrical; horizontally oriented; and have a total length of 14m, a diameter of 3m, and a thickness of 12mm. The insulation material that covers the surface is rock wool and has a thickness of 40mm. The tanks may not be effectively stratified because there is no system to help the phenomena and the tanks are installed horizontally. This is the reason why the tanks are considered to be more of energy buffers than energy storage.

The maximum amount of energy stored in the tanks can be calculated using Equation 4.3 and the minimum supply temperature and normal generation values expressed in Table 4.4. The heat loss can be estimated with the help of the work presented in [116].

E_stored =ρ_wV_storedc_pw(T_g−T_f) (4.3)

Table 4.4: Storage installed in Parc Bit

Type Volume Generation T. Supply T. Return T. Energy

Heating 200m³ 89^◦C 75^◦C 60^◦C 3,248kWh_t

Cooling 200m³ 6^◦C 8.5^◦C 12^◦C 580kWh_c