WSHP System Control and Optimization 5: Design Tips for Maximum Efficiency

By Chad Edmondson

Engineers have many options for optimizing energy efficiency in a WSHP system. Most of these opportunities involve making the most of the part load conditions since these systems almost never operate at peak load. Some are useful strategies regardless of load conditions.

Making the Most of Part Load Conditions

Specify WSHPs with a 2-stage scroll or variable speed compressor. These types of compressors not only save energy, they are less likely to short cycle and therefore provide better temperature and humidity control.

Use diversity factors to determine the peak “block load” for the system and size the equipment (WSHP, cooling tower and boiler) based on this value rather than the overall peak load. Virtually all commercial buildings have enough demand diversity to reduce the need for capacity. This is especially true of WSHP systems which are designed to recover and transport energy to wherever it is needed.

Specify cooling towers or closed circuit coolers with smaller variable speed fans, and large boxes for greater heat transfer area. The reduction in fan horsepower will more than make up for the slight increase in cost over the life of the system.

Choose a condensing boiler if at all possible. Condensing boilers, while more expensive than non-condensing boilers, are perfectly suited for the extended part load conditions associated with WSHP systems. Not only are condensing boilers more efficient at part load, they thrive on the low return water temperatures that a WSHP system produces.

Do not oversize zone WSHPs! Doing so will undermine efficiency and create humidity issues due to the frequent on/off cycling that will occur as a result of sensible cooling loads being met before latent load.

Use parallel 50/50 variable speed pumps instead of a single large pump. Two smaller pumps manifolded together are more efficient than a larger pump at part load. Plus, you get a significant amount of built-in redundancy.

Specify WSHPs with a modulating hot water reheat option. A WSHP with a dehumidification/reheat option can often provide cost-effective dehumidification, and sometimes help owners avoid the cost of a dedicated outdoor air system (DOAS) or energy recovery unit. An internal pump circulates warm condenser water through the hydronic reheat coil during the “cooling with reheat” mode of operation, warming the dehumidified supply air before it enters the space. Modulating reheat can provide up to 100% “neutral” 72°F supply air even at part load (non-design) conditions.

Additional Advice for Optimizing Efficiency

In addition to the above strategies, consider specifying a heat pump water heater to heat domestic water for the building. These units often have a COP of 5 or greater, and it makes more sense to use BTUs rejected from the cooling system to heat water than it does to reject them through the cooling tower. A hot water storage tank can be added to “store” BTUs for the water heater throughout the day so they don’t go to waste.

We also suggest that designers use an automatic flow regulating balancing valve to control flow through the WSHP. Based on the required delta T for the unit, these valves prevent overflowing of the WSHP. If the system is variable flow, consider installing an automatic balancing valve on each pump. This helps ensure proper water through the heat pump (when the compressor is operating) as the system flow rater changes.