In the previous article I analysed the annual energy consumption of a hypothetical commercial building formed by 6 offices, a conference room and a small fleet of 6 electric commercial vehicles.
Now, it's time to make it a zero carbon building, trying to unplug it from the electric grid and power it with renewable energy sources.
At this stage I am not interested in practicalities on how we build the power system in details such as how the different electricity generators will coexist. This will be a task for a detailed electric system design and it will be addressed at a later stage.
One of the main goals of this project is to build a commercial building that can be classified as a Level 6 building by the Energy Saving Trust.
According to the Energy Saving Trust "Code for Sustainable Homes" publication, a Level 6 building needs to achieve a 100% reduction in CO2. To reach zero carbon, all emissions produced for space heating, water heating and lighting must be zero or negative.
In addition to this, all CO2 emissions from cooking and domestic appliances must be offset. The required offset is based on floor area. This offset can be achieved with micro-generation, other onsite generation or offsite generation provided that this is connected directly to the development via a private wire arrangement.
In our case, as explained in previous articles, the techniques adopted to achieve this outstanding result will be as follows:
- Passive House standard to reduce the energy required for space and water heating
- Solar thermal panels for water heating
- Photovoltaic (PV) panels and wind turbines for onsite electricity generation
The total amount of energy required by the building, which we set to 216000 KWh/year, will be thus produced onsite and, in addition to that, a fleet of 6 electric cars will be powered by the same onsite micro-generation system.
A part of the electricity will be generated by a number of solar panels installed on top of the roof. The flat available area on the roof is 600m2. Let's allocate 400m2 for PV panels and 200m2 for solar thermal panels.
A Kyocera KD210GH-2PU 210W high efficiency multicrystal photovoltaic module occupies an area of 1500mm x 990mm, which is equivalent to 1.485m2. We can arrange about 270 panels on a flat 400m2 roof surface, providing a total peak power of 56.7KW.
The nominal peak power is the power rating given by the manufacturer of the module or system. It is the power output of the modules measured at 1000W/m2 solar irradiance. This means that if the modules were 100% efficient, we would need 1 m2 to get a system with a peak power of 1kW.
Since the modules are not 100% efficient and not all locations have a 1000W/m2 solar irradiance (some have less and some have more than that), we need to take into account all the losses and the actual solar power at a specific location across a year time.
The European Commission web site provides a useful tool to calculate the performance of grid-connected photovoltaic modules, given a European location and some information about the PV panels.
For the location, I have taken as an example Guildford, a town in Surrey, South England (51°14'24" North, 0°35'6" West, Elevation: 56 m a.s.l.). The parameters used for the estimation are as follows:
- Nominal power of the PV system: 56.7 kW (crystalline silicon)
- Estimated losses due to temperature: 7.7% (using local ambient temperature)
- Estimated loss due to angular reflectance effects: 2.9%
- Other losses (cables, inverter etc.): 14.0%
- Combined PV system losses: 22.9%
The result for a fixed system (with no sun-tracking mechanism in place) with an optimal inclination of the panels of 36° is shown in Table 1 (graphical representation in figure 1).
Table 1 - Performance of Kyocera 210W solar panels at Guildford, Surrey, UK
where:
Ed: Average daily electricity production from the given system (kWh)
Em: Average monthly electricity production from the given system (kWh)
Hd: Average daily sum of global irradiation per square meter received by the modules of the given system (kWh/m2)
Hm: Average sum of global irradiation per square meter received by the modules of the given system (kWh/m2)
This shows that the total estimated annual energy yield is 50000 KWh (just for comparison, the same PV system in Sevilla, South Spain, would yield 80700 KWh/year. It is not difficult to understand why!).
Figure 1 - Monthly performance of an array of Kyocera 210W solar panels at Guildford, Surrey, UK
Figure 2 - Horizon height at Guildford, Surrey, UK
Since I have set our electricity requirement to 216000 KWh/year, it still remains to cover 166000 KWh of energy. A large part of it can be generated by wind turbines.
To calculate better how much electricity we can generate out of a wind turbine it's important to estimate the average wind speed at the building site. It's possible to find out the average wind speed for a specific British location by converting a UK postcode to an Ordinance Survey grid reference and then obtaining the wind speed from the wind speed database of the British government Department of Energy and Climate Change.
For the same location near Guildford that we used to estimate the electricity generated by solar panels, the results are as follows:
- Wind speed at 10m asl (in m/s): 4.2 - 5.5
- Wind speed at 25m asl (in m/s): 5.1 - 6.2
- Wind speed at 45m asl (in m/s): 5.6 - 6.7
Assuming that we choose a location above 25m asl, we can take 6 m/s as the mean wind speed for our calculation. Probably this might be a generous assumption, but let's stick to this value, bearing in mind that the results will be just a rough estimation of the real world.
Now that we have an idea of how much wind blows over our heads, let's have a look at some turbine models on the market. If we decide to use just one medium size turbine, we need to look for something in the range of 50-100 KW of peak power.
A three blade Coemi Skywing 50 KW turbine generates an average annual yield of 120 MWh of electric energy, according to its data-sheet.
A two blade WES18 80KW turbine (figure 3), manufactured by Wind Energy Solutions, a Dutch company, has an annual electricity production of 161 MWh with a mean wind speed of 6 m/s or 102 MWh/year with 5 m/s.
Figure 3 - Power curve of a 2 blade WES18 wind turbine
We need to take into account that usually the rated figures published by the manufacturers are slightly higher then the actual energy production. For example, the power curves tend to be better then the actual measured performance.
Let's assume that using a WES18 wind turbine we can generate 120MWh in a year. These figures are pretty much confirmed by the Danish Wind Industry Association Wind Power Calculator, so this assumption should be quite safe to make.
Another option would be to use an array of small wind turbines. Quietrevolution is a London based company that designs and produces an elegant model of small vertical axis turbines. They claim that at an average wind speed location, their 5m high qr5 turbine can generate 7500 KWh/year. This means that to generate the same energy as we can generate with the 2 blade WES18, we need about 16 turbines, which would give 120 MWh/year. Or we could try to ask for a number of bespoke qr12 turbines which they say is a product still under development but potentially available for a number of selected pilot projects. One 12m high qr12 would deliver 45-55 MWh/year, so using two of them in conjunction with three qr5 we would make up what we need.
Figure 4 - Quietrevolution qr5 wind turbine
The Quietrevolution option may seem odd at first - why should we want to use many small turbines rather then just one bigger? The answer is: they are much shorter (a WES18 has a hub height of 32m, whereas a qr5 is 5m high), more elegant and nice to see and thus they may have less impact on the landscape. But the actual choice will be made considering many variables, not last the cost of the installation.
With any of the above solutions it would still remain to generate about 40 MWh/year of electric energy.
This last chunk of energy could be generated by additional small wind turbines, or other PV solar panels, maybe on the roof of a car park. We may build a 300m2 roof for the car park and install 210W PV solar panels on top of it. But again the final decision is to be made considering many different factors, like the actual average wind speed as well as the economic impact that a specific choice of energy mix can have on the project.
I think we can leave this remaining 40MWh/year not quite defined at the moment and we will decide what the best solution is depending on the actual circumstances.
We should also remember that this 40MWh would be mainly to produce more electricity then what is likely to be actually needed. Although in the previous article I estimated the building energy consumption to be about 120000 KWh/year, I eventually set the energy requirement to 216000 KWh/year. Even if it is not strictly required, it would be good to have this energy buffer in order to be totally independent from the grid and possibly sell electricity and make a profit.
Combined Heat and Power (CHP) technologies have been deliberately left out of the list of possible energy sources. This is because I would like to try to abandon any kind of fuel, even if it may possibly be renewable. Having to burn bio-fuels still means to be dependent on someone else that produces this fuel and hence on the price fluctuation of this product on the market. For the scope of this project we would rather try to stay far from any kind of fuel and stick to "high value" renewables, which are those that come completely for free from the nature: wind and solar power.
Some projects across the UK have been realized by massively using CHP as a solution for the energy demand. We believe the high quality of Zero Emission Workspace resides in the fact that we want to demonstrate that burning fuels is not strictly necessary and that to achieve a “real” zero emission target this should be actually avoided. We say “real” because someone argues that bio-fuels are zero-carbon renewable resources, since for each gram of CO2 emitted burning them, there should be a bio-fuel plantation somewhere that absorbs the same amount of CO2. But this rationale doesn’t take into account the impact that bio-fuels have on the agriculture and the energy required to harvest, stock, process and transport the fuel. Obviously, to build and transport wind turbines and solar panels doesn’t come for free, but the impact of this industrial process is much less.
Now that I have gone through the possible options to power a zero carbon commercial building, there are two main topics that remain to analyse more in depth: how we can cope with wind and solar power fluctuations and how much all of that can eventually cost.
Follow me to the next article.
