Designing net zero energy buildings for new construction and for renovation of existing buildings presents a diverse set of challenges. It requires analyzing the unique energy use of the entire building and then designing a system that can reduce the net energy footprint without sacrificing functionality or comfort. And as the net energy is reduced, onsite renewable energy plays a larger role in efforts to reach the net zero energy goal. The type of heating, ventilation, and air-conditioning (HVAC) system selected must offer fundamental characteristics. It must 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 most importantly, easily connect to onsite renewable energy systems. While this may sound complex, a water source heat pump (WSHP) system can provide solutions.
And by including non-HVAC equipment into the system design, it can actually reduce the initial cost of construction.
Energy is transferred into and out of the water loop for use throughout the building by way of uninsulated water piping that connects individual WSHPs to meet the expected cooling and heating load of each temperature zone. 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 building. And 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 65°F 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 of this system revolves around three processes of the net energy water loop system: the means to remove waste heat from the loop to maintain the temperature range of the system and reuse that energy elsewhere in the building; the means to add heat to the loop from sources of waste energy or from sources of renewable energy; and the ability to improve the operating efficiencies of other components within the building.
The first process of removing waste heat from the net energy water loop can 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, you are 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. But in commercial buildings and in hotel applications, these domestic hot water systems normally experience long periods of time with no flow 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 will offset the costs for implementing this optimization strategy, and at the same time move 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, adding passive heat of rejection to existing grey water and black water piping in the building uses significantly less energy than running a fluid cooler.
The second process of adding heat to the net energy water loop could be accomplished with a boiler. However, this should be the last stage of adding energy to the loop, because a boiler uses additional offsite energy, which increases the energy footprint of the building. Even in northern climates, most buildings produce enough waste heat to completely eliminate the need for boilers.
One optimization strategy is mentioned above: moving energy from the exhaust air system into the net energy water loop. This recovered energy can be used by the WSHP system and the domestic hot water system. Several other strategies to recover waste heat throughout the building to maintain the minimum temperature range of the net energy water loop are available from non-HVAC systems and from renewable energy options described below.
The third process of increasing the efficiency of other equipment in the building ties into this second process of adding heat to the net energy water loop. Water-cooled refrigeration and ice-making equipment can also help building’s achieve zero net energy. Instead of noisy, low-efficiency, air-cooled refrigeration cases, freezer cases, and ice-making machines, water-cooled machines can be used for about the same cost. And the water-cooled versions are 20% more energy efficient than the air-cooled versions, which reduces the building’s energy footprint. The HVAC cooling load is reduced for that zone, which reduces the building’s total energy usage, and the heat rejected by these water-cooled versions is a reliable source of recovered heat to the net energy water loop.
Now that a net energy water loop system has been optimized, onsite renewable energy can be easily connected. Onsite renewable energy from hot water solar panels can add heat into the net energy water loop. An onsite cogeneration plant can also add heat into the net energy water loop while generating electricity. Onsite solar photovoltaic (PV) panels will have a greater impact on achieving net zero energy because the diversified part-load operation of the net energy water loop reduces peak demand.
Designing a net zero energy building and incorporating onsite renewable energy sources can seem complex. But selecting a WSHP system as the backbone for a building-wide net energy water loop allows for the application of a wide variety of optimization strategies unique to the building. A WSHP system can provide a seamless connection to onsite renewable energy as well as a simple path to a net zero energy design.
A version of this was previously published in Construction Specifier Magazine.