Thursday, November 27, 2008

Net-Zero Energy

Today I want to follow up a little bit on the net-zero concept I described in yesterday’s post on the Pearl River Tower.

How Does Net-Zero Work?
Net-zero energy buildings (ZEBs) produce as much energy on-site as they use annually. The reason ZEBs are referred to as "net-zero" is that they are still connected to the grid. Sometimes they are producing more power than they are consuming and feeding power to the grid and running the meter back, and sometimes they are consuming more power than they are producing and pulling power from the grid. But for a ZEB, the energy given to the grid is equal to the amount of energy pulled from the grid on an annual basis.

Net-Zero Energy ≠ Carbon Neutral
It’s important to note that net-zero refers to energy use, and does not necessarily mean zero carbon emissions from energy use. A key issue here is what type of energy the building produces. For tall buildings, renewables probably will not be enough to power the entire building, and therefore some sort of carbon-based fuel source will have to be used.

The Pearl River Tower, for example, gets less than 10% of its energy needs from the wind turbines and BIPV. The designers planned to use natural gas microturbine co-generation for a significant portion of its electricity generation (although this ultimately was scrapped). Now, of course using natural gas-powered co-gen is not carbon neutral, but is incredibly efficient, generally in the range of 60-80% (versus ~45% for super high-efficiency coal plants). Given both the high-efficiency nature of this process and the relatively low level of carbon in natural gas, this type of process has the potential to significantly reduce emissions over drawing electricity from a predominantly coal-powered grid. So, net-zero-energy is not necessarily carbon neutral, but a really good intermediate step.

However, renewables could provide enough power to get to net-zero energy use. Low-rise buildings with large, flat roofs are ideal siting grounds for PV panels and therefore are easier to bring to net-zero carbon. As renewables become more efficient, they will also increasingly be able to power taller buildings.

But there is also the issue of embodied energy, which as I mentioned in my post a few days ago, accounts for a significant portion of a buildings lifetime carbon emissions (20% in the West, 40% in China). So just being operationally net-zero-energy does not mean that a building is carbon neutral when looking at the entire lifecycle, but brings us a whole lot closer to where we need to go in terms of energy use and carbon emissions.

Opportunities for ZEBs in China
The following excerpt of a report by the US Nat'l Renewable Energy Lab (PDF) describes the feasibility of implementing ZEBs on a wide-scale in the US:
Using today’s technologies and practices, the technical potential is that 22% of the buildings could be ZEBs. With projected 2025 technologies, the technical potential is that 64% of the buildings could be ZEBs. If excess electricity production could be freely exported to the grid, then with the projected 2025 technology in every building, the commercial sector could generate as much as 37% more energy than it consumes.
Good news for the US built environment, and there is reason to suspect that there is an even larger opportunity in China. The conclusions of the NREL report only apply to existing buildings, but achieving net-zero is much easier in new construction. Since China will double it's floor space over the next 15 years, the Chinese have a great opportunity to implement net-zero designs on a massive scale. This isn't simply pie in the sky, since as the Pearl River Tower proved, net-zero design is already possible in China (although policy may have to change in some cases to accommodate running the meter back).

Moreover, the NREL report singled out warehouses as the best opportunity for ZEB penetration, estimating that fully 95% of warehouses could be ZEBs by 2025, versus less than 25% for offices (although as I mentioned above I think China can beat this level thanks to their large amounts of new construction). China's logistics market is (or at least was) hot, and grew at nearly twice the rate of GDP in the first half of this decade. While such rapid growth is probably not great from a green viewpoint, the fact is, this has created a lot of flat roof space, which can be viewed as a good opportunity to install a lot of PV and get a large number of Chinese buildings to net-zero.

But if Chinese owners are to pursue this opportunity, there are still hurdles to overcome. First and foremost is the fact that grids in places like Guangzhou do not yet have the infrastructure for net metering. Hopefully JUCCCE's smart grid program can help overcome this road block.

Second is the financing model for solar PV, the technology that NREL thinks is the best way to get to net-zero. Solar is expensive and many developers will be unwilling to spring for the expense. An alternative financing model needs to be developed. One promising scheme is the "rent-a-roof" model, where building owners lease their roof space to utilities who then install solar panels. I think this is a fantastic idea, since not only do building owners not have to pay for the solar panels, they actually get paid to rent their roof out! That's a real no-brainer. Utilities own and install the solar panels and use generate electricity for the grid. This will probably take some good policy to get it to scale, but pairing this financing scheme with a smart grid could allow China to really make progress on building CO2 emissions.

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