The overall objective of this proposal, EW-201725, is to demonstrate an innovative approach for retrofit military buildings that improves the energy performance and resiliency of a packaged rooftop Heating, Ventilation, and Air Conditioning (HVAC) unit by directly powering it with a solar photovoltaic (PV) array and utilizing direct current (DC) power. HVAC typically accounts for 30-40% of a commercial facility’s energy usage and is directly attributable to triggering a customer’s peak demand when energy prices are high. The demonstration of a DC rooftop HVAC system will offer the Department of Defense (DoD) an attractive HVAC retrofit approach that increases the energy efficiency and provides a direct means for DoD facilities to further reduce their energy costs during peak hours while also providing improved energy resiliency. This system will be widely replicable and can benefit all types of military buildings that utilize an HVAC system. Although this DC microgrid demonstration is for a retrofit application, it is also easily incorporated into new construction projects.

Technology Description

This demonstration will connect a new rooftop HVAC unit to an existing DC microgrid utilizing a PV array and infrastructure previously installed with Environmental Security Technology Certification Program (ESTCP) funding (project EW-201352). The HVAC unit's compressor(s) and fan(s) will be connected to the DC bus and powered by the solar PV array. The variable speed compressors and fans operating on the DC bus can be controlled and adjusted to various optimum speeds of operation to meet the facility needs unlike the current off-the-shelf AC rooftop HVAC systems. There are a number of hours during the day when a building may not be fully occupied or the need for cooling is lower than designed, however, the cooling rate of AC rooftop HVAC units cannot be adjusted easily with a constant speed system, which are typically only capable of being turned completely on and off or only have very few stages of adjustment. Having the DC HVAC system powered by solar PV removes the need to convert the solar PV energy from DC to AC with an inverter and then back from AC to DC for utilization in the HVAC motor drives, resulting in further efficiency gains. As a result of connecting the DC HVAC unit onto Bosch’s DC microgrid, this system becomes resilient with the capability to operate in an emergency mode during a grid outage as needed by the facility.

Implementation Issues

Based on the experience from ESTCP project EW-201352, which converted commercial lighting to operate on DC using solar PV on the same building, the benefits that can be achieved by this system is an anticipated overall system efficiency gain of 7- 10% when powered by solar PV and lifetime cost savings. In addition, pairing this system with solar PV generation and its ability to optimally control the speed of the compressors and fans, energy and peak usage can be reduced and better regulated correlating to lower energy consumption and cost. Connecting the HVAC unit onto Bosch’s existing DC network will also provide higher reliability, as it removes conversion equipment which is the frequent point of failure. Resiliency is improved as well, if the DoD chooses to use the system for enhancing building comfort and safety during an emergency situation.