Water-loop heat pumps and net-zero

Boiler-Tower-¬
Typically, a boiler is employed to maintain closed loop temperatures above 15.5 C (60 F), and a cooling tower to maintain loop temperatures below 32 C (90 F). These systems are applicable in medium to large buildings regardless of whether the load is heating- or cooling-dominated.

Adding heat to the loop
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 has already been mentioned—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 in this article.

OpenLoop-¬
In ideal conditions, an open-loop application can be the most economical type of geothermal system. It utilizes a well, lake, or ocean as a direct energy source.

Improving other components
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. Greatly underused is the expansion of the net-energy water loop to also include refrigeration and ice-making equipment. Instead of noisy, low-efficiency, air-cooled refrigeration cases, freezer cases and ice-making machines in cafeteria kitchens, restaurants, coffee shops, mini-marts, and gift shops, water-cooled refrigeration cases, freezer cases and ice-making machines can be used for about the same cost.

These water-cooled versions are 20 percent more energy-efficient than the air-cooled versions, reducing the building’s energy footprint. The HVAC cooling load is also reduced for that zone, which lowers the building’s energy footprint. Further, the heat rejected by these water-cooled versions is a reliable source of recovered heat to the net-energy water loop.

Now that the system has been optimized, onsite renewable energy can be easily connected—energy from hot-water solar panels can add heat, as can an onsite cogeneration plant (while also 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 electrical demand.

Conclusion
Designing a net-zero-energy building seems to be a complex task at first—effectively adding onsite renewable energy can create another hurdle. Fortunately, selecting a low-first-cost, simple-to-install/operate water-source heat pump system as the backbone for the net-energy water loop allows for the application of various optimization strategies unique to the building. With a seamless connection to on-site renewable energy, the WSHP system provides a relatively simple path to a net-zero-energy design.

PondLoop-¬
This system is very economical to install when a large body of water is available. Coils of pipe or a submerged heat exchanger are simply placed in the pond or lake.
VertLoop-¬
The ideal choice when available land surface is limited. Well-drilling equipment is used to bore small-diameter holes from 30 to 120 m (100 to 400 ft) deep.

 

 

 

 

 

 

 

HybridLoop-¬
This assembly combines boiler tower and geothermal loop technologies to achieve optimal efficiencies, while meeting the challenges of both loop field space and budget constraints.

Alan Niles is western region sales manager for the commercial sales group of WaterFurnace International. He is an active member of the American Society of Heating, Refrigerating, and Air-conditioning Engineers; and is an ASHRAE Distinguished Lecturer. For more than 25 years, he has been presenting at local, regional, and national trade shows to provide technical assistance to architects and engineers. Niles is a member 
of the International Ground Source Heat Pump Association, and holds a bachelor’s degree in mechanical engineering from the University of Oklahoma. He can be reached at 
alan.niles@waterfurnace.com.

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