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article_Hawxhurst_Pollman.jpg

©2017 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 59, no. 12, December 2017

By Kevin Hawxhurst; Joshua Williams; Anthony Pollman, Associate Member ASHRAE; Anthony Gannon, Associate Member ASHRAE

About the Authors
Kevin Hawxhurst and Joshua Williams are masters students; and Anthony Pollman and Anthony Gannon are assistant professors at the Naval Postgraduate School, Monterey, Calif.

Replacing or supplementing a building’s heating and cooling energy requirements with renewable-powered alternatives is attractive, particularly in areas with vast renewable resources. However, regardless of the selected renewable generating method, the resources are usually intermittent. In the case of heating and cooling applications, thermal storage is an attractive and readily available technology to overcome this intermittency.

To integrate onsite renewable energy and thermal storage, a novel control strategy was implemented in the microgrid system at the Naval Postgraduate School’s (NPS) Integrated Multi-Physics Energy Laboratory (IMPEL). By coupling wind and solar power generation with a control strategy that matched load demand to power generation, renewable energy (alone) was used to charge building heating and cooling thermal storage systems using an isolated microgrid. This system holds promise as both a renewable-only-based microgrid or a grid-tied system where local renewable generation is available.

The traditional electric grid provides energy to customers by matching power generation to demand. Renewable energy resources struggle to fit the current model due to intermittency, despite their potential to generate large amounts of power.1 As more renewable energy sources become grid-tied, more large-scale energy storage is required to mitigate the variability in renewable energy production. In heating and cooling applications, thermal loads are the main consumer of energy with the pumping of fluids, either air or water being smaller. Thermal energy storage is already commonly used for grid-tied applications and is therefore attractive for storing renewable energy where heating and cooling are the main loads.

The control theory and systems developed here can be used in two ways. The first would be in a traditional grid-tied system where local renewable energy is available and the desire is to use as much of this energy as is available. A reason for this may be that there are variable electrical rates through the day. A second reason would be the desire to have a very secure system with the ability to isolate this system from the main grid using a microgrid. The control system was designed to charge the thermal storage systems by matching their load to the available renewable supply. These thermal stores could then be used by the building control system at a later stage to offset energy consumption from either the grid or in the case of isolated systems, batteries or generators.

A microgrid, or localized electric grid, usually includes a combination of power generation sources, storage devices, and loads, all of which can be grid-isolated. This independence allows microgrids to provide unique energy solutions, especially through the use of renewable power generation such as solar and wind. However, typical microgrid systems still apply the traditional energy management approach of matching power generation to load demand through the use of supplemental fossil-fuel-driven power generators. The need to provide power during peak demand, coupled with the intermittent nature of renewable energy resources, means that current microgrid systems are still dependent on electrical storage and supplemental power generation that are sized to meet this peak demand. However, adding thermal energy storage devices can reduce the size of generators and electrical storage devices such as batteries.

 

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