Optimal control of cooling towers
The design of cooling towers is usually based on worst-case conditions: warm and humid ambient conditions. When the ambient air is colder/drier than design conditions, or when the actual cooling need is lower than the design value, an overcapacity is available. This can be used to optimize the energy consumption by reducing the pump- and fan speeds in the cooling tower, or even better by lowering the water temperature in the cooling tower, to improve the func-tion of the machine cooled by the tower.
Compressor cooling system get a better energy efficiency with lower cooling water temperatures, and since the electricity consumption of the compressor system is far bigger than that of the cooling tower, a change in the water temperature will have the largest effect.
When cooling needs or ambient conditions change, finding the optimal operating point is not a simple matter. Often, the controllers use simple fixed temperature setpoints instead of using the opportunities to reduce water temperature.
Likewise, compressor systems and cooling towers are often supplied by different subsuppliers, each with their own idea of how to control the individual units. Very few have a well-founded advice on controlling the combined units in an energy optimal way.
The goal of the present project is accordingly to define and test energy optimal control algo-rithms for various combinations of cooling towers and their "customers"
The aim of the project is to improve the control of cooling towers in order to save energy. In contrast to many other projects, this project attempts to optimize both cooling towers and the machinery connected to the cooling towers. The logic behind this is that the energy consumption of the machinery is often linked to the temperature of the cooling water from the cooling towers. In other words, if one only looks at the cooling tower and tries to optimize its energy consumption, this may lead to an increased consumption in the connected machinery. So, to obtain the really minimum energy consumption, a more holistic approach is needed, where the energy consumption of both cooling tower and its "clients" is taken into account.
The "clients" are typically compressor refrigeration systems, where the condenser on the refrigeration system is cooled with water from the cooling tower. A low water temperature leads to a low condensing temperature and thereby a low energy consumption on the compressors. To obtain a low water temperature, however, the cooling tower fans must run fast, leading to a high energy consumption on the fans. The goal is thus to find the optimum operating point for the fans, which will give the lowest total energy consumption.
The main content of the project is thus to design and test a control procedure that minimizes the overall energy consumption of cooling towers and "clients".
The conclusions drawn in the project so far can be summarized as following:
- In theory, considerable energy savings can be obtained by optimal control of cooling towers and their clients, especially in cases where the cooling towers and clients are operated at low loads and/or low ambient temperature.
- An optimization algorithm has been designed, based on a simple mathematical model of the cooling towers.
- Many industrial operations run full load 24 hours a day, in order to make the best possible use of the production machinery. In that case, there should still be a possibility for optimization, when the ambient air temperature is lower than the design temperature.
- In the case of compressor refrigeration systems, the optimization process will be limited by the fact that most compressor systems require a certain minimum condensing temperature to ensure a sufficient pressure to supply refrigerant to all consumers. In that case, the cooling tower fan speed will be controlled by the minimum condensing temperature, and no further optimization is possible.
- In the case of evaporative concentrators, it has not been possible to prove any optimization based on reducing the cooling water temperature. Data indicate that capacity actually decreases, as cooling water temperature is lowered. This is not logical in any way, and the reason for this behavior has not been identified.
- A number of project partners already have fan speed controllers attempting to minimize the total energy consumption. Such controllers are available for use on "simple" systems, meaning systems where all cooling towers run in parallel at the same fan speed, effectively acting as 1 big cooling tower.
- The analysis suggested that considerable energy savings were attainable for one site, to be confirmed by on site tests.
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| DAKA | ||
| Vestas Industrial Cooling | ||
| ARLA FOODS AMBA | ||
| ARLA FOODS AMBA | ||
| Arla Brabrand | ||
| Region Syddanmark | ||
| Ørsted A/S |
