One of BedZED’s sustainability objectives was reduced environmental impact. Some of the original community plans included zero fossil fuel, 100% renewable energy use, zero heating homes, passive solar heating, PV power for 40 electric vehicles, wind-powered ventilation systems, low embodied energy materials, recycled timber, reused structural steel, and urban tree waste bio-fueled CHP [4] (Figure 2). For example, the buildings were designed to be energy efficient that stored heat when it was warm and released heat when it was cold so as to eliminate the need for central heating; a green transport plan was also implemented to minimize the residents’ use of vehicles requiring fossil fuels, as well as the need to commute to work [2] .

BedZED was designed to be carbon neutral, meaning it should generate enough renewable energy on-site for heating, hot water, and electrical uses. BedZED’s design to rely only on solar, wind, and biomass energy was successful up to a point. Some of this can be attributed to homeowner education and willingness to change lifestyles and reduce energy use, in conjunction with energy-efficient and hot-water saving appliances. Homeowners were reported to use 45% lower electricity and 81% less hot water than the borough average [5] . 777 m² of photovoltaic (PV) panels line the rooftops and south facing second floor windows [5] . Wind cowls on the roof harness the wind to create a healthy, well ventilated internal environment [6] .

Materials used in the construction of BedZED were carefully selected for low environmental impact, so that the building materials were sourced locally, reclaimed, and/or recycled whenever possible. For example, all wood used was approved by the Forest Stewardship Council to guarantee they came from a sustainable source. This approach during the construction stage is reported to have reduced the environmental impact of BedZED by 20% - 30% [6] .

One of the main sources for renewable energy in BedZED was designed to be generated by a custom biomass combined heat and power (CHP) plant using local wood fuel to generate both heat for hot water and electricity. The CHP plant operated by having woodchips fed into a dryer, which operated on waste heat from the generator engine. The woodchips were then transferred to a gasifier, where heating them converted them into a combustible gas (gasification process). The gas was then cleaned, cooled, mixed with air and fed into a spark ignition engine (Figure 3) [5] .

Although the CHP plant was operational for a while, it now lies abandoned. The new, experimental equipment required constant modifications and full time maintenance. Unforeseen issues such as the reliability of equipment, tar condensing from the wood gas, and staffing requirements to operate led the plant to be shut down

[5] . The costs required to run such a small scale plant were simply unfeasible. Because the CHP is no longer in operation, hot water for the community is produced by a gas condensing boiler, and most electricity is supplied by the national grid; the PV panels only supply about 20% of BedZED’s demand [5] .

Study results show that although residents in BedZED are unable to achieve a one planet living level ecological footprint, and even with the inoperable CHP plant, the energy consumption in the homes is much lower than the UK average [5] [7] . While it may not have met the original goals, BedZED was still an important step in the right direction towards a sustainable future; the positive results of BedZED led BioRegional to publish a Toolkit for Carbon-Neutral Developments [7] .

This Toolkit is a practical guide on producing carbon neutral developments that minimize environmental impacts while still remaining profitable, including technical descriptions, monitoring results, and financial mechanisms that allowed BedZED to become a reality [7] [8] . Of special note is the BedZED Construction Materials Report that goes into detail about every aspect of construction in terms of its environmental impact. It covers the extracting, processing, and transporting of construction materials and explains how to deal with construction waste [7] .

Masdar City’s power infrastructure includes a mixed system of renewable energy options, such as a “range of photovoltaic plants (PV), a concentrating solar thermal power plant (CSP), evacuated thermal tube collectors, and a waste-to-energy plant” ( [12] , p. 3953). For instance, consider how the city manages hot water. Masdar City’s hot water is usually provided by photovoltaic electricity; in addition, solar collection through evacuated tubes has also been used to support the hot water when needed. Moreover, geothermal heat has been tested as an optional use in cold weather seasons. Masdar City is the home of the largest photovoltaic plant among the Middle East, generating up to 10 MW from two main modules, a polycrystalline photovoltaic module and thin film photovoltaic module; these serve the Masdar administration and institute buildings, as well as provide energy for the onsite construction. A 1 MW solar photovoltaic plant on the roof of the Masdar Institute provides some of the solar power in this city, with the remaining energy coming from 88,000 photovoltaic panels on a solar farm [10] .

However, there is a prediction that the energy needs will be increased along with the growth of this city. That is to say, the innovation of these renewable energy technologies will be hard-pressed to keep up with the expanded community energy use [13] [14] . In order to maintain its sustainable low energy waste, multiple renewable energy projects are being considered to provide this city with clean energy sources.

Masdar City plans include mixed-use and low-rise buildings. The buildings have been designed in several ways to implement the zero energy system. 95% of the roofs have been installed with solar panels, and 90% of the buildings have green roofs. 85% of the building roofs have both solar panels and green roofs [12] . These two com- ponents not only generate energy but also regulate temperature and provide shading for pedestrians (Figure 5).

While not fully completed yet, the early buildings constructed here are plentiful with sustainable ideas. For example, in the Masdar Institute, sandstone inwardly slopes on the building facades, which allow the light to go through into the building. In addition, the sandstones function as building insulation blocks and maintain the building’s comfort level [9] [11] . Photovoltaic panels on the roofs power the buildings, and a central tower displays the energy use [9] . Auditorium and cafes built in this building provide the building’s mixed use and creates convenience for students [9] . Other major building technologies include interior natural ventilation, floor heat strategies, and shading strategies. All these concepts reduce building energy demand and help create buildings with low energy systems in long run.