Historic Building Gets Net-Zero Energy Overhaul

For over fifty years, Iowa engineering firm MODUS has been designing MEPT (mechanical, electrical, plumbing, and technology) systems for new and existing buildings.

A year ago the company purchased a century-old structure in the historic Market District of Des Moines and began a $17M renovation project to turn the former warehouse into a state-of-the-art net-zero energy building. MODUS moved its offices to the newly modernized facility in December of 2014, and uses it as both its home base and a showcase of energy-efficient design and technology.

The updated structure, now known as “Market One,” is Iowa’s first commercial net-zero energy building. MODUS is applying for LEED Platinum certification, which addresses factors such as energy efficiency, indoor air quality, sustainable building materials, and water conservation.

HVAC

Climate control is provided by a geothermal heating and cooling system that consists of eighteen vertical loop geothermal wells, each drilled to a depth of 380 feet. Vertical loops tend to be more expensive than horizontal loops, but in urban settings they’re usually the only option, since horizontal loops require a lot of land.

The ground sourced heat pump feeds a variable refrigerant flow (VRF) HVAC system, a very efficient method of transferring heat in older buildings that have limited ductwork. VRF allows individual control of each space, making it possible to heat one area while simultaneously cooling another. Heat exchangers connected to the ventilation system assure that outgoing indoor air transfers its heat (or its “cool,” so to speak, depending on the season) to the incoming air, reducing energy loss while maintaining the proper number of air exchanges per hour.

Lighting

An abundance of windows provides plenty of natural daylighting, assisted by efficient LED lights whose intensity and color spectrum changes throughout the day, mimicking the natural circadian light patterns. LEDs automatically dim according to the amount of daylight reaching the room.

Photovoltaics

Market One generates its own electricity with a pair of photovoltaic arrays. The building sports a car canopy with 676 solar panels that combine to produce 189 kW (peak). On top of the building itself, there’s a PV array consisting of 111 panels that generate up to 31 kW. On a sunny day, the PV arrays provide roughly one MWh of electricity - twice as much energy as the building will use in a typical day.

Both arrays use non-tracking PV modules that are tilted to optimize production in the summer, similar to the Bullitt Center in Seattle. Net-zero energy doesn’t mean off-grid - it just means that overall the building produces more energy than it consumes. Market One will generate excess energy in the summer; that energy is sold to the grid. In the winter the building will purchase grid power.

String Inverters vs Microinverters

One thing I thought was unusual is that the canopy array is tied to a pair of large string inverters while the rooftop array consists of microinverters - one inverter per PV module. Each type of inverter has its own strengths and weaknesses, and MODUS engineers decided to take advantage of both kinds in their design.

With a string inverter, groups of panels are wired in series to maximize voltage and reduce current, with each string feeding one input of the inverter. This has two benefits: first, one or two string inverters for a whole array is less expensive than one microinverter for each panel. Second, the string inverter can be placed indoors, away from the elements and extreme temperature fluctuations. However, when one panel on a string is shaded, it can drastically reduce the power output of the entire string. This is because a PV cell is a light-controlled current source, and panels wired in series experience the same current. The total current in each string is limited to its weakest performer. Shading a small fraction of an array could result in a significant loss in total power. In the image below, when one panel is partly shaded, its current goes from 5A to 1A, which limits the entire string’s current to 1A. The array that should produce 228W now generates only 46W - almost an 80% reduction in power!

Microinverters are used when partial array shading is an issue. Because each panel has its own inverter, its power output is independent of the performance of other panels. In the picture below, you see that the same shading issue only reduces power by 19%. (In both examples I’m using round numbers and approximations for simplicity.)

It turns out that Market One’s canopy array is unshaded during peak sun hours, so it made sense to use a string inverter for that array. The rooftop, however, includes a patio with a pergola, which partially shades the array at certain times of the day. Rather than eliminating the outdoor living space or dealing with the inevitable power loss, designers selected a microinverter system for the rooftop array.

Besides the shading issue, microinverters allow a PV system to be gradually scaled up. With a string inverter, the designer must know how many panels will be in the array, since the panels are wired together before reaching the inverter. Panels with microinverters can be added to the array at any time, so if it’s desirable to start small and add to the array as funding becomes available, microinverters are the way to go. In this case it wasn’t funding, but city criteria and potential changes in the size of the rooftop deck that forced the engineers to deal with an unknown array size during the system design.

Everything is a trade-off, though. Microinverters cost more per watt than string inverters, and because they’re physically attached to the panels, they’re exposed to the weather, which could shorten their lives.

Cost and Payback

The renovation came with a $17M price tag; about $2M of that went for the energy upgrades, some of which was instantly recovered by renewable energy credits and incentives. MODUS expects the green technology to pay for itself in less than a decade - not bad for a building that’s stood for over a hundred years. A quick glance at MODUS’ portfolio shows that the company has renovated a wide variety of buildings, bringing many of them up to LEED standards. You might say that energy efficient design is its modus operandi.