Net-zero Yoga Barn: Wind Turbine and Direct Exchange Geothermal
West Tisbury Net Zero Residence – description
This mixed use residence has a large yoga studio on the first floor and living quarters above.
Built around the frame of a reclaimed barn (originally built in 1852), this structure features an extremely tight and energy efficient building shell. As one of Martha’s Vineyard’s two net-zero buildings powered by wind turbines, a 5 kW wind turbine provides enough power annually to meet the entire energy load. Walls (R50), ceilings, and floors (R60) were insulated to reduce the size of the mechanical system. It was carefully designed to meet the projected load of 20 people practicing yoga.
This project demonstrates the value of careful design and building shell optimization. By focusing on long term energy use (as driven by the selection of a 5 kW wind turbine and its projected output) and the goal of annual net zero operation, the building shell was improved and the mechanical system reduced until energy goals were met.
A horizontal 6 ton direct exchange geothermal field was selected to serving radiant space heating loads through a buffer tank. (picture shows excavation and copper tubing – this pit was then refilled with soil removed during excavation.)
The small site footprint precluded the use of a closed loop horizontal glycol geothermal system so we opted for the direct exchange approach. This reduced our field size by half which reduced excavation costs as well. (picture shows grid of 1/4″ copper tubing in the field)
The geothermal heat pump uses refrigerant R407 as an improvement over R22’s ozone depletion potential. It has a 6 ton capacity to meet the radiant heating and domestic hot water loads.
DHW preheat is accomplished using a flat plate heat exchanger powered by the heat pump with final hot water heating from an electric water heater.
Walls, ceilings, and floors were much thicker and better insulated than the norm – this worked well to reduce the sensible loads but increased the need for mechanical ventilation. We installed an ERV (energy recovery ventilator) large enough for the nominal load of 5 people exercising – we understood that the occasional larger classes would present an intermittent load that could be met over a longer period of time by a smaller ERV. An ERV is connected to a duct system designed for a future dedicated outdoor air
Use of a buffer tank for the radiant heating system with the six ton heat pump ensured that more of the operating time of the heat pump would be closer to full load, especially when a smaller radiant zone was the only active zone. A separate outside reset on the buffer tank optimizes heat pump operation; the other two areas each have separate reset temperatures as the different rooms have differing loads.
We set up an outside reset scheme for the buffer tank to make sure heat pump energy use was held to a minimum – the buffer tank setpoint typically rides about 5F hotter than the outside reset setpoint of the other two radiant zones.
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