The Passive House standard, as promoted by the Passive House Institute (PHI) achieves highly energy efficient buildings whose demand for heating and cooling energy can be reduced by as much as 90% compared to conventional builds. This is achieved using an envelope-first approach and considers the five principles of Passive House: airtightness, high-quality windows, climate-applicable insulation, avoidance of thermal bridges and sufficient ventilation. This results in an extremely low operative demand, and thus minimal operative emissions, in Passive House buildings; making them a perfect match for renewable energy sources, which can be met with challenges of supply and demand (peak loading), weather conditions and overall market uptake.
Inspired by the urgency of meeting global climate change goals, the Passive House Institute initiated a review of primary energy use in buildings in 2013 to better reflect synergies between energy efficiency and the use of renewable energy resources. 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 and is quite commonly used as an environmental indicator to assess the energy performance of buildings. Recognising the fact that this approach was developed for a fossil fuel-based energy supply structure and needs updating, PHI developed an adapted primary energy concept.
PHI was very aware that non-renewable forms of energy use by buildings needed to be rapidly phased out. As such, they devised a method to incentivise the use of renewable forms of energy in buildings. Their research resulted in the development of the Primary Energy Renewables factor and the introduction of two new classes of the Passive House Standard: Passive House Plus and Passive House Premium.
Primary Energy Renewable ‘Factors’ and how they work
All the new Passive House classes are now calculated using Primary Energy Renewable (PER) factors. These are designed to encourage the efficient use of renewable energy sources and create incentives for installing mechanical equipment compatible with a renewable energy supply. For example, using a heat pump water heater to produce hot water will result in a lower Primary Energy requirement than using a gas tank water heater, making it easier to meet the certification target.
Renewable primary energy (PER) is the unit of energy generated from renewable resources, e.g. electricity produced by a photovoltaic system/wind turbine or heat generated with a solar thermal system. PER-factors reflect the primary renewable resources needed to cover the final energy demand of a building, including distribution and storage losses. In the case of a PER-factor of e.g. 1.5, a surplus of 50% renewable primary energy is needed in order to be able to meet the final energy demand at the building. The higher the PER-factor, the higher the required renewable energy resources and therefore the more important the implementation of efficiency measures in order to avoid compensation from non-renewable sources.
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 vary for different energy uses and can also vary from city to city.
The heating demand for domestic hot water and for household electricity feature fairly constant demand profiles over the course of the year. The demand can be covered to a large extent directly from the primary energy source, without the need for storage, or via efficient short-term storage technologies. This results in PER factors of about 1.3. The energy demand for heating, on the contrary, only occurs during winter with lower renewable energy resources. A large part of the energy demand must, therefore, undergo seasonal storage, which implies high losses. As an example: For Central Europe, the PER-factors for electric heating are around 1.8. This higher factor clearly indicates the increased importance of employing efficiency measures to reduce the heating demand.
The difference between Net-Zero and Passive House Plus/Premium
Conventionally, calculations of net-zero depend on the difference between a building’s annual energy demand and annual on-site renewable energy production. These calculations not only neglect energy storage losses but also penalise 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 space. The Passive House approach deviates from these traditional methods for crediting renewable energy supply to buildings, recognising that all sites are not created equal in this regard. It uses the following principles:
- Renewable offsets are calculated as a function of Projected Building Footprint (PBF) rather than total floor area. PBF is more proportional to the available roof area than total floor area, which means multi-story buildings may achieve the Plus and Premium standards.
- Buildings with no solar access on-site may purchase off-site renewable energy facilities to achieve Plus or Premium certification.
- PH ‘Classic’ Buildings with no on-site or off-site renewable energy supply are still optimised for efficiency first and a future grid supply of all renewable energy.
While biofuels are considered a renewable energy source, they carry a penalty for replacing food production. Their burning also generates particulate matter that is both unhealthy and emits carbon. For these reasons, the use of biofuels is allowed but has been capped to limit its use.
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. Many contributions are being made 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, while higher classes encourage the achievement of even lower renewable primary energy demand and additional renewable energy generation. Already, the Classic standard ensures that buildings are optimised to become batteries themselves: they’ve been proven to retain an unprecedented level of thermal comfort while eliminating peak loads. This optimisation 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 inadequate comfort for lengthy periods of time without any active energy inputs. This quality offers economic benefits to both the utilities and microgrid 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 opportunities are boundless.