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Introduction

Andreas Athienitis, William O'Brien, and Josef Ayoub

1.1 Evolution to Net-Zero Energy Buildings

Buildings have evolved over time from largely passive systems into structures with increasingly high levels of environmental control, partly through the addition of man-made insulation materials, such as fiberglass and polystyrene. The adoption of electric lighting in early twentieth century buildings, contributed to a reduction in window areas and reliance on artificial lighting, particularly in the period from 1950 to 1970. But in the 1980s, the development and acceptance of sealed double-glazed windows with an insulating airspace, or insulating windows with special coatings to reduce heat transfer and optimize transmission of solar radiation (Athienitis and Santamouris, 2002), led to the adoption of larger fenestration areas (up to 60% of the façade area) in both the residential and commercial buildings. These large fenestration areas - as much as 90% of the façade area - lead to high heating and cooling energy consumption. Thus, fenestration and daylighting significantly influence the design of commercial buildings. The drivers of the design of residential buildings are shifting from space conditioning to appliances, lighting, and integrated energy systems, as building envelopes and HVAC become more efficient and passive techniques are employed.

Since the early 1990s the potential of solar radiation incident on building surfaces to satisfy all their energy needs has contributed to the idea of net-zero energy buildings gaining widespread acceptance as a technically feasible long-term goal (for most regions). A net-zero energy building (Net ZEB) is normally defined as one that, in an average year, produces as much energy (electrical plus thermal) from renewable energy sources as it consumes. When the energy production is on-site the Net ZEB definition is most strict.

The visible part of the solar spectrum (nearly half of total solar radiation) is useful as daylight. Almost all of solar radiation can be converted to useful heat for space heating, as well as other useful purposes, such as heating water and drying clothes, or even solar cooling using passive and active solar systems (International Solar Energy Society (ISES), 2001). Another solar technology - photovoltaic (PV) - that converts solar radiation to electricity has recently experienced significant advances and dramatic reductions in cost (almost 90% cost reduction per watt of generating capacity in the last 10 years). Both technologies can be integrated and optimized for combined heat and power generation to advance buildings toward net-zero energy consumption.

Most inhabited areas receive significant amounts of sunshine that enable the design of technically feasible Net ZEBs with current solar and energy efficiency technologies. For example, in Canada between latitudes 40-53 °N where most of Canada's population lives, a suitably oriented façade or roof on a typical building receives up to ~6 kWh/m2 per day, and the incident solar energy often exceeds total building energy consumption. Photovoltaic panels integrated on the roof and façade can typically convert 6-20% of the sun's energy into electricity, and 50-70% of the remainder can be extracted as heat from the PV panels, while 10 to 30% can be utilized for daylighting in semitransparent systems. Combined solar energy utilization efficiencies on the order of 80% can be achieved if proper integration strategies are implemented and nearly the full spectrum of solar radiation can be utilized as daylight, useful heat, or electricity.

The energy generation function in Net ZEBs using solar energy - as daylight, useful heat, and electricity - requires a transformation of the way buildings are designed and operated so as to be cost effective and affordable. The key challenges for smart Net ZEBs to overcome are summarized in Table 1.1 for each of the four major building subsystems where the current situation is contrasted with the expected characteristics of Net ZEBs. In addition, the integration of design with operation is considered.

Table 1.1 Challenges for smart Net ZEBs

1.1.1 Net ZEB Concepts

The convergence of the need for innovation and the requirement for drastic reductions in energy use and greenhouse gas (GHG) emissions in the building sector provides a unique opportunity to transform the way buildings and their energy systems are conceived. Demand abatement through passive design, energy efficiency, and conservation measures needs to be simultaneously considered with integration of solar systems and on-site generation of useful heat and electricity using a whole building approach.

Building energy design is currently undergoing a period of major changes driven largely by three key factors and related technological developments:

1. The adoption in many developed countries, and by influential professional societies, such as ASHRAE, of net-zero energy [3] as a long-term goal for new buildings;
2. The need to reduce the peak electricity demand from buildings through optimal operation, thus reducing the need to build new central power plants that often use fossil fuels; and,
3. The decreasing cost of energy-generating technologies, such as photovoltaics, which enables building-integrated energy systems to be more affordable and competitive. This is coupled with increasing costs of energy from traditional energy sources (e.g., fossil fuels).

A key requirement of high performance building design is the need for rigorous design and operation of a building as an integrated energy system that must have a good indoor environment suited to its functions. In addition to the extensive array of HVAC, lighting, and automation technologies developed over the last 100 years, many new building envelope technologies have been established, such as vacuum insulation panels and advanced fenestration systems (e.g., electrochromic coatings for so-called smart windows), as well as solar thermal technologies for heating and cooling, and solar electric or hybrid systems and combined heat and power (CHP) technologies. A high-performance building may be designed with optimal combinations of traditional and advanced technologies depending on its function and on climate.

Solar gain and daylight control through smart window systems, in which the transmission of solar radiation can be actively controlled, remain a challenge in building design and operation because of the simultaneous effects on instantaneous and delayed heating/cooling loads, and on thermal and visual comfort. Solar gains may be controlled through a combination of passive and active measures - with the passive measures employed during design and active measures, such as positioning of motorized venetian blinds during operation. Since solar gains have delayed effects because of building thermal mass, there is significant benefit in predictive control and optimal operation of passive and active storage that utilizes real-time weather prediction (Athienitis, Stylianou, and Shou, 1990).

New building technologies, such as phase change materials (PCM), active façades with advanced daylighting devices, and building-integrated solar systems, open up new challenges and possibilities to improve comfort and reduce energy use and peak loads, and they need to be taken into account in developing optimal control strategies. The energy requirements and control needs of commercial and residential buildings are usually quite different. For example, in commercial buildings, cooling and lighting play major roles, while in houses, especially in cold climate regions, space heating and domestic hot water heating dominate energy consumption.

Plug loads (e.g., due to appliances and office equipment) represent a large portion of building energy consumption and their share is increasing, as HVAC and lighting systems become more energy efficient. Demand response strategies, such as scheduling of appliances, are becoming more popular as a way to significantly reduce the impact of plug loads on peak electric demand.

1.1.2 Design of...