The existing water loop heat pump (WLHP) loop is under very basic control. When the temperature of the loop reaches too high a temperature, the evaporative cooler operates to bring this temperature down. When the temperature of the loop drops too low, the boiler turns on.
This operation of the loop temperature results in the following:
|•||The loop temperature is at its lowest during net heating mode (when most of the heat pumps are heating). This is relatively inefficient mode of operation, since heat pumps (when heating) are most efficient with a high loop temperature.|
|•||The loop temperature is at its highest during net cooling mode (when most of the heat pumps are cooling). This also is relatively inefficient, since heat pumps (when cooling) are most efficient with a low loop temperature.|
We recommend installing controls to schedule the building loop based on building operating mode (net heating or net cooling).
When the building is clearly in net cooling mode, the temperature of the loop should be brought as low as possible. This temperature will be dictated by how much heat can be rejected by the cooling tower, especially during very warm temperatures (The cooling tower is able to bring the temperature down only as low as the outside wet-bulb temperature). This strategy will reduce the overall electrical consumption, since the heat pumps in cooling mode will be working to overcome a lower temperature "lift" between the heat source and the heat sink. The operating COP (coefficient of performance) will be higher.
When the building is clearly in net heating mode, the temperature of the loop should be heated to 95F. This will reduce the overall electrical consumption, since the heat pumps in heating mode will be operating more efficiently.
There will be times when the occupancy and outside temperature changes will cause the building to switch between net cooling mode and net heating mode over the course of a single day. When this occurs, optimizing the loop temperature to maximize heat pump savings would be offset by the extra energy required to change the loop temperature. The controls must be programmed to select a median temperature and avoid frequent loop temperature changes when the building is switching frequently between modes.
Install controls to optimize the loop temperature under all operating conditions.
The controls needed to make loop systems efficient. Takes a lot of thought to get it right. Often missing in existing systems.
Issues and Concerns
This measure will require either a special-purpose pre-programmed controller or an unusually capable DDC programmer and a very precise specification of the control logic. A typical DDC system programmer working with a general specification is unlikely to make this work properly.
Analysis text here