by Alan Niles
Designing net-zero-energy buildings for new construction or renovation presents many challenges. It requires analyzing the unique energy use of the entire facility and then designing a system that can reduce the net-energy footprint without sacrificing functionality or comfort. As energy consumption is reduced, onsite renewable energy plays a larger role in efforts to reach the goal of net-zero energy. Water-source heat pumps (WSHPs) can be important in this respect.
The type of HVAC system selected must offer fundamental characteristics. It needs to:
- provide comfort by independently heating and cooling individual temperature zones;
- capture and transport waste energy throughout the building;
- be scalable for any size building;
- readily connect to, and share energy with, a wide range of other HVAC and non-HVAC systems within the building; and
- easily connect to onsite renewable energy systems.
A WSHP system can meet these criteria while also being very simple to design, install, operate, and maintain. By including non-HVAC equipment into the system design, it can actually reduce the first cost of construction.
Understanding WSHP systems
Using uninsulated water piping to connect the individual WSHPs that have been selected to meet the expected cooling and heating load of each temperature zone, energy is transferred into and out of the water loop for use throughout the building. A WSHP in the cooling mode will move heat from the local conditioned space into the water loop, while a WSHP in the heating mode will move energy from the water loop into the local conditioned space. As each WSHP operates independently, the net-energy water loop system is completely scalable to any size of building. Further, the system increases in efficiency during part-load operation.
In the most basic configuration, the net-energy water loop system operates without any additional transfer of energy while the water loop temperature ranges between 18 and 38 C (65 and 100 F). As fewer units cycle on at any given time, less energy is required to maintain this temperature range.
With this basic system in place, the path to net-zero energy becomes simple. Optimization revolves around three processes:
- means to remove waste heat from the loop to maintain the temperature range of the system and reuse that energy elsewhere in the building;
- means to add heat to the loop from different sources of waste energy or from sources of renewable energy; and
- ability to improve the operating efficiencies of other components within the building.
Removing waste heat from the loop
The first process of removing waste heat from the net-energy water loop could be accomplished with a basic fluid cooler or cooling tower. However, this should be the last stage of heat removal because additional offsite energy is being used by the fluid cooler, and the energy removed is not recovered for use elsewhere in the building. In short, one is paying money to get rid of usable energy.
One optimization strategy for reusing waste heat connects the domestic hot water system to the net-energy water loop system. Adding storage tanks in the domestic hot water loop would allow a water-to-water heat pump or a heat recovery chiller to move energy from the WSHP water loop system into the domestic hot water system. Increasing the water volume of the domestic hot water system with larger storage tanks significantly reduces the size of the water-to-water heat pump and associated equipment required to move energy to the domestic hot-water system.
Domestic hot-water systems are generally sized for a recovery time based on peak water flow usage. However, in commercial buildings and in hotel applications, these domestic systems normally experience long periods with no or little flow. During this time, a very small water-to-water heat pump can move an immense amount of energy out of the WSHP system to preheat enough hot water in storage tanks to meet the large volume of hot water required by a hotel during morning showers. Eliminating large boilers from the domestic hot-water system and its associated energy consumption offsets the costs for implementing this optimization strategy, and, at the same time, moves the building closer to net-zero energy operation.
Another optimization strategy for reusing waste heat involves connecting the outside air system and the exhaust air system to the net-energy water loop system. A water-to-water heat pump can move energy out of the WSHP system and into the outside air system for pre-heating the make-up air. When there is no waste energy in the WSHP system, the water-to-water unit or a six-pipe modular heat recovery chiller with simultaneous hot water and chilled water production from a single compressor can take waste heat from the exhaust air system and reuse that energy for pre-heating the make-up air. It also adds energy to the net-energy water loop, if needed by the domestic hot-water system.
Should the amount of recovered heat exceed the building’s need for energy and the building’s ability to store the heat for later use, then the addition of passive heat of rejection to existing greywater and blackwater piping in the building uses significantly less energy than running a fluid cooler.1