Solar Arrays and Net Zero Buildings

I attended a presentation on an office building designed to meet the Living Building Challenge. One of the design concepts was to achieve net zero energy (metered) for the completed design. This means that the metered power sent to the utility grid from the building equals or is greater than the power required from the utility grid.

The project had some inherent difficulties such as a small site, the need to build to a maximum floor area ratio (FAR), and limited southern exposure. The design relied on a building envelope optimized for energy performance. To achieve net zero, photovoltaic panels were required to offset the building power demand from the utility grid. The array formed a parasol hovering above the roof, extending well beyond the building walls to gain the area needed for power generation.

How to Get to Net Zero
The energy analysis diagrams showed an annualized net zero power consumption. The maximum power was generated during the summer months; the minimum during winter (no surprises from these data). The summer solar power exceeded the building demand allowing power to be sent to the utility grid. During winter, the building required power from the utility grid to make up for the reduced generation from the solar array.

A design team major concern was the connected plug load. After optimizing other systems, the plug loads represented a larger percentage of the total electrical load. The design team reported results of their study that showed computer loads could be significantly reduced by using energy efficient monitors and CPUs. Demand could be reduce even more by relying on cloud computing. In effect, cloud computing moves processing off-site, shifting the load to a remote centralized computing location.

Achieving net zero energy is a good thing - right?
Certainly reducing power consumption is good. The more we can do to minimize the required power, the less demand there will be to construct new generating capacity. Since most utility generating stations are driven by fossil fuels (coal, fuel oil, and natural gas), reducing the demand reduces the fuel required. By burning less fuel, less polluting carbon dioxide will be emitted to our atmosphere.

If the goal is for net zero, does transferring a load from the new building to another existing remote building really achieve the ultimate goal? The new building design results in net zero. The remote building inherits the computing load shed by the current building and increases its dependence on the utility grid. The power need was not eliminated or even reduced. It was simply moved from one location to another.

DC vs. AC - The Current Battle
Because Thomas Edison lost his battle against Westinghouse in the 1880s, the United States uses alternating current (AC) instead of direct current (DC) for power distribution between generating stations and ultimate electrical loads. AC power proved much more efficient for long distance transmission. So AC power won out, despite Edison's best efforts to preserve the importance of his DC related patents.

Solar arrays generate direct current power. Plug loads require alternating current. Therefore photovoltaic solar arrays require inverters to convert the generated DC power to usable AC power. Inverters require power to operate. As a result, the power produced by the solar array is not 100% efficient. Plus the solar panels, themselves, are only 12 - 15 percent efficient. Taking into account the entire system, the combined efficiency may be as little as 10 percent.

Electronic Equipment Loads
Consider our typical electronic equipment loads.  Every computer, every monitor, and every mobile device charger has a transformer to convert AC power to DC power. These transformers also consume energy. So when we use solar arrays, DC power is generated, converted to AC power for distribution, and then converted back to DC to drive our electronic devices.

If the solar array could power electronic devices directly, there would be no loss through inverters and transformers. Essentially 100% of the generated power could be available for end use. Plus, the building HVAC system would not need to be designed to remove the heat produced by both the inverters and the transformers.

As I sit here in my San Diego hotel room writing this blog, I look around. My iPad, cell phone, and laptop are all plugged in to a receptacle through a transformer. The two flat screen TVs and digital alarm clock conceal their transformers within their cases. One room, six DC devices powered from an AC system, all through individual transformers. The other lug loads are four incandescent light fixtures, three compact fluorescent light fixtures, an iron, and a coffee maker. All of these are capable of being powered by DC current.

Let me tell you, I would gladly give up hauling my Dell power brick (a power supply the size of a brick, only heavier) everywhere I go with my laptop if I could simply plug in to a DC outlet.

Was Edison Right After All?
With our ever increasing dependence on electronics that rely on DC power, maybe Edison had the right idea after all. Maybe we need to consider DC power for consumption and AC power for distribution. Then we could capture the greatest efficiency from solar arrays and eliminate all the transformers used by every electronic device.

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