Our All-Renewable Energy Future
Here’s why the overhaul of the Passivhaus standard resulted in two new certification levels, Plus and Premium
If you’ve been puzzled by the proliferation of "net," "nearly" and "almost ready" zero-energy definitions and standards and have wondered just how net or nearly they truly are, take heart. The Passivhaus Institut (PHI) has introduced an equitable assessment of energy use to help guide us toward the 100% renewable energy future we must rapidly achieve.
Inspired in part by the impressive leaps in the efficiencies of renewable energy generation, coupled with the urgency of meeting global climate change goals, PHI initiated a review of non-renewable energy use in buildings in 2013. It recognized their previous calculations for primary energy needed updating, especially as they favored the use of natural gas over electricity. (Primary energy accounts for all the source energy used by a building, including the amount of energy it takes to generate and transmit power to the building site.)
PHI recognized that non-renewable forms of energy use by buildings needed to be rapidly phased out, so it devised a method to incentivize the use of renewable forms of energy in buildings. The research resulted in the overhaul of the existing Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. "Classic" standard and the introduction of two new standards: Passive House Plus and Passive House Premium.
Primary energy renewable factors and how they work
All of the new Passive House standards now calculate primary energy using Primary Energy Renewable (PER) factors. These are designed to encourage the use of renewable energy sources and create either incentives, or disincentives, for installing various types of mechanical equipment in Passive House buildings. For example, in San Francisco, using a heat pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed. to produce hot water will result in lower primary energy requirement numbers than using a gas-fired tank water heater would, making it easier to meet the certification target. (A heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. water heater has a PER factor of 1.25 versus the 1.75 factor for a gas-fired water heater.)
PER factor calculations are based not only on fuel source, but also on site-specific load profiles calculated on an hourly basis. In this way, variations in regional utility grid source energy and typical time-of-day use profiles (which impact the availability of renewable energy to meet a utility’s load) for the local climate and region are factored into these calculations.
As a result, the PER factors can vary from city to city in California (see image #4 below). For example, the electricity PER factor for heating demand via heat pumps is 1.80 in Sacramento. This relatively high PER factor incentivizes reducing heating demand in winter, when renewable energy supplies are low. In San Diego the comparable PER factor is set at 1.30, where the climate is milder and cooling is typically a greater peak load issue.
Crediting renewable energy equitably
Conventionally, calculations of net zeroProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. Calculating net-zero energy can be difficult, particularly in grid-tied renewable energy systems, because of transmission losses in power lines and other considerations. depend on the difference between a building’s annual energy demand and annual on-site renewable energy production. These calculations penalize tall buildings with small roof areas, buildings with no solar access, or buildings that opt to use their roof area for green space or as active living spaces.
PHI took a major deviation from such traditional methods for crediting renewable energy supply to buildings, recognizing that all sites are not created equal in this regard (see image #2 below). PHI’s approach uses the following principles:
1) Renewable offsets are calculated as a function of Projected Building Footprint (PBF) rather than total floor area. PBF is more proportional to available roof area than total floor area, which means multi-story buildings may achieve the Plus and Premium standards.
2) Buildings with no solar access on site may purchase off-site renewable energy facilities to achieve Plus or Premium certification.
3) PH "Classic" buildings with no on-site or off-site renewable energy supply are still optimized for efficiency first and a future grid supply of all renewable energy.
Biofuels, micro-grids, and battery storage
While biofuels are considered a renewable energy source, they carry a penalty for replacing food production. Burning biofuels also generates particulate matter that is both unhealthy and emits carbon. For these reasons, the use of biofuels is allowed, but has been capped.
The most intriguing areas of innovation with regard to manifesting the 100% renewable energy future currently look to be in developing our capacity to store renewable energy (see image #5 below). We’re excited by the contributions being made right here in California to develop technologies that are contributing to our new energy future. Existing storage capacity from hydroelectric schemes is now being joined by a growing array of affordable short- and long-term battery storage options. Converting renewable energy into methane gas is another rapidly developing technology that could increase the viability of renewable energy by allowing us to store it for longer.
Remarkably, these options are all currently supported by the Primary Energy Renewable calculations embedded in the Plus, Premium, and Passive House Classic standards. Indeed, the Classic standard at the heart of all of them remains the foundation that most equitably supports an all-renewable energy future. The Classic standard ensures that these buildings are optimized to become batteries themselves: They’ve been proven to retain an unprecedented level of thermal comfort while eliminating peak loads.
This optimization ensures that even without the addition of active power, their passive capacity is what is literally doing the heavy lifting. These buildings enable occupants to survive in adequate comfort for very lengthy periods of time without any active energy inputs. This quality offers economic benefits to both the utilities and micro-grid designs of renewable energy storage systems that extend well beyond comfort. Just imagine what we could do with renewable energy if we didn’t need so much of it to simply operate buildings? The possibilities are boundless.
Bronwyn Barry is a Certified Passive House Designer and the co-president of the North American Passive House Network. This post was first published in Passive House Buildings: California's Energy Future. Additional articles and California project examples are available in the free e-book and PDF here.
- Images #1, #2, and #3: Bronwyn Barry
- Images #4, #5, and #6: Passive House Institute
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