Local Power Distribution with Nanogrids
Power distribution in buildings has hardly evolved in the last 100 years. We have improved safety (circuit breakers, grounding, indulation), and added the occasional UPS, but little else. However, we could create new technology that would apply digital control and Internet principles to power distribution to enable useful capabilities we lack today. We could have “plug-and-play” integration of local generation and storage, off-grid operation (when the utility grid is too expensive or absent), and optimal use of each energy source. This technology would also enable direct use of local renewable power in DC devices—saving about 10 percent of electricity over the common conversion to and from AC power. The technology that can do all this is Local Power Distribution based on nanogrids.
Power distribution in buildings has hardly evolved in the last 100 years. We have improved safety (circuit breakers, grounding, indulation), and added the occasional UPS, but little else. However, we could create new technology that would apply digital control and Internet principles to power distribution to enable useful capabilities we lack today. We could have “plug-and-play” integration of local generation and storage, off-grid operation (when the utility grid is too expensive or absent), and optimal use of each energy source. This technology would also enable direct use of local renewable power in DC devices—saving about 10 percent of electricity over the common conversion to and from AC power. The technology that can do all this is Local Power Distribution based on nanogrids.
Matching electricity demand to supply is a major challenge for a future world of renewable sources that are inherently variable. Buildings will increasingly consume—and supply—multiple types of power, both AC and DC. Each power source will have different levels of reliability and cost, which may continually change. Examples include DC power from rooftop PV (variable supply but low marginal cost), AC power from the grid (reliable supply but with higher and increasingly variable cost), and battery power (finite supply with replacement cost).
With this increasing complexity at the “edge” of the grid, total central control becomes infeasible, so new approaches are needed. Some applications lack grid connectivity, sometimes or always, but still need to match supply and demand and so need to operate correctly whether grid-connected or not.
The most basic mechanism to determine the quantity and timing of commodity use is price. Price-based allocation for electricity needs to be fully explored before more arbitrary or complex solutions are developed. We propose a technology of “local power distribution”, which is modeled on Internet principles, to implement digitally-managed electricity distribution within buildings. Buildings will typically be a microgrid, and often contain a network of “nanogrids”.
We define a nanogrid as a single domain for voltage, price, reliability, quality, and administration. It has loads, may include storage, and has gateways to the outside (local sources, other local grids, or the utility grid). Nanogrids use price to mediate local electricity supply and demand, optimizing electricity allocation, by providing the correct signal of local scarcity — this may not be possible to do through any other mechanism.
Figure 1. Two connected nanogrids
Figure 1 shows two connected nanogrids. The controller receives requests for power from loads and grants or revokes such requests. The controller sets the local price takes into account the quantity and price of any external electricity it has access to, internal demands and storage conditions. It uses this price both within the nanogrid and in negotiating exchanges across gateways. Controllers will buy or sell power across gateways whenever it is mutually beneficial as indicated by relative price.
Nanogrid loads take the local electricity price into account in deciding how to operate, along with functional considerations. High prices will usually reduce or delay energy use; low prices increase or advance it. Battery storage is optional, but can increase reliability and stability.
Nanogrids implement power distribution only—they perform no functional control of the devices connected to them. Separating power distribution and functionality into distinct “layers” is a key principle of local power distribution. In the future, devices that need to coordinate functionally, such as those in the same room, will often be powered differently, and devices that share a power infrastructure may have no functional relationship. The layered model lets each evolve separately, greatly simplifying the development of new technologies and their deployment alongside existing products.
Nanogrids are a bottom-up means of evolving the power distribution system. Analogous to the Internet paradigm, the utility grid is the core backbone network and nanogrids are the Local Area Networks that connect to each device. Technologies in wide use today, such as Power over Ethernet (PoE) and USB already provide digitally-managed power distribution, as does the developing IEEE Universal Power Adapter for Mobile Devices standard (P1823, UPAMD).
Nanogrids can be a universally useful technology, for any building type — residential, commercial, industrial, vehicles, etc. A key application is developing countries where electricity is often much more expensive in absolute and relative terms than elsewhere, so that it is even more important to optimally manage supply and demand, as nanogrids provide for. In a village which today has little or no electricity, in future each household could have a nanogrid, with an arbitrary link topology between them to enable optimal sharing, with generation and storage scattered among the households.
Nanogrids have inherently security advantages because communications for power distribution are only between entities with a direct power connection. Nanogrids also enable privacy because other grids don’t need to know details of the devices or their use within the nanogrid.
A next step is to build hardware and software platforms that implement nanogrid technology to demonstrate its feasibility and efficiency, and to test out policies for controller management and for load behavior.
Local power distribution complements technologies that improve the utility grid, and by removing complexities of coordinating with individual devices in buildings, will make those simpler and more effective. It can provide local operational management with lower costs and reduced energy use.
For additional information on nanogrids see IEEE Computer, September 2012, p. 89, and http://nordman.lbl.gov
