In Paolo Bacigalupi’s science fiction novel set in Thailand somewhere in the next century, energy is stored by mechanical means. Giant elephant-like creatures are harnessed to millstones, which elevate the tension on miniature circular springs, winding them tighter and tighter. Reduced to the size of a hockey puck, kilowatts of energy are released by the controlled uncoiling of the spring.
Those of us in the semiconductor industry tend to focus on the process of converting the three-phase 480 VAC entering our offices, factories and data centers down to 48 VDC, then to 12, 5 and 1, ideally never losing a watt. We take for granted the solar energy farms laid out on the roofs of homes and offices, the micro-inverters that increase solar energy efficiency and the granularity with which we can monitor the panels. We focus on the tail end of the energy harvesting process ― the consumption of electricity ― and less on its generation, capture, and storage.
We suspect something is missing with the rapid adoption of solar energy, the proliferation solar panel farms, and the ongoing reductions of cost-per-watt. The majority of solar installations remain connected to the utility grid. That means the utility grid supplies power to a solar home when the sun sets, or when it is obscured by rain or fog. The use of a giant battery or supercapacitor is forfeited. While is on-again/off-again relationship to the electric utility grid allows home owners to take just what they need ― even sell back their surplus ― academics are furtive about the prospect of running a society on batteries. They are exploring methods of long-term energy storage.
We are already familiar with high-density multi-cell lithium-ion batteries which release hundreds of watts from batteries. We are even learning to charge them wirelessly, to parallel multiple lithium ion cells for use in electric cars, power tools, or warehouse-sized data centers. With solar inverter developments and solar energy storage mechanisms, we become familiar with higher energy sources in increasingly smaller form factors. What do we know about energy storage? Can we think about higher voltages and currents? Storing energy efficiently will require new technology investments.
Zero-to-60 on 900 amperes
Tesla’s roadster, an all-electric vehicle which accelerates zero to 60 MPH is a matter of seconds, consumes 900 amperes in the process. Battery technology becomes a key diver for energy storage and transmission. In Siemens’ vision, electric cars connected with cabling in a parking garage can top off their batteries, or sell any surplus energy back to the utility company running your local grid.
Investors see Tesla’s giant battery factory and its partnership with Panasonic as the nose of a “secular change,” in which energy storage as a major piece in the world’s energy puzzle.
The company’s visionary founder and CEO, Elon Musk has seen the need for “vertical integration across EVs (electric vehicles), solar panels and battery energy storage.” The company’s home energy storage unit, the Powerwall 2, represents an attempt to apply what it’s learned from electric car batteries to batteries for the electric home.
Meant to interface between the utility company grid and the main lines to a home, the Powerwall includes its own inverter for converting DC from the battery the 220 or 120 used by home. According to Tesla’s promotional literature, the Powerwall 2 stores enough energy to power an average two-bedroom home for a full day. For longer storage periods, multiple Powerwall units can be stacked’
Tesla’s home electrical storage unit has a 14-kWh capacity, enough to power a small home for a day. It delivers 5 kW under typical household loading, or 7 kW peak.
Harness electricity from chemical reactions
Much of the current research on low battery storage life uses what we still call fuel cells, in which electricity is produced by extracting hydrogen from water. One technology showing commercial potential are flow batteries. ESS (Energy storage systems Portland) offers a good example of what it calls an Iron Flow Battery (IFB). It uses an iron-based electrolyte, along with salt and water (FeCl2, KCI, and H2O). The long duration storage is good for time-shifting renewable energy tasks, ESS claims ― meaning you can store energy for specific tasks at a future date (like running a big data center computation). You don’t have to use it on the day you collect it. The IFB has a lifespan that exceeds 20,000 charge and discharge cycles. It has quick-response power electronics, supporting microinverters, which can perform voltage and frequency regulation on microgrids or utility-scale applications.
The flow battery technology by ESS is based on an electrochemical ferrous/iron plating reaction on the negative side and the ferrous/ferric redox (reduced oxidation) reaction on the battery’s positive side.
The ESS battery will store up to 400 kWh, and output 50kW AC. The output of the battery (the input to offices or factories) is 480 VAC (three-phase). Its container is 40 feet long, 8 feet wide, and 9 ½ feet high ― about the size of a tractor trailer truck.
Interestingly, the storage capacity ― the number of watts per cubic foot ― is almost the same as Tesla’s Powerwall; roughly 125, as you can see below.
ESS’ Iron Flow Battery and Tesla’s Powerwall have the same Power Density
Building a solar energy installation in the Pacific Northwest
In 2009 Washington State University (WSU) published an advisory for installing a residential solar energy harvesting system, asking readers to make choices among components for systems with energy storage, and systems with no significant storage. WSU included assumptions of the costs per watt of electricity extracted, based on photovoltaics. In 2008, for example, the installed cost of a residential PV system in the United States ranged from $8 to $10 per installed watt before government or utility incentives.
Studies by Washington State University examined grid-connected systems examined grid-connected AC systems with no battery or generator back-up, and grid-connected AC system with battery back-up.
Source: Washington State University “Solar Electric System Design, Operation and Installation,” October 2009.
Basic components of grid-connected PV systems ― with or without batteries ―include Solar photovoltaic modules, grounding equipment, emergency disengage switches, inverters, and power meters
Among PV module types, single-crystalline (“mono-crystalline”), poly-crystalline and amorphous silicon (“thin film silicon”) were among the most widely used. Other cell materials used in solar modules were cadmium telluride (CdTe). In 2005 approximately 90% of modules sold in the United States were composed of crystalline silicon, either single-crystalline or poly-crystalline. The market share of crystalline silicon was down, WSU reported, from previous years due to increased use of amorphous thin film silicon.
On rated power, grid-connected residential PV systems use modules with rated power output ranging from 100-300 watts. Modern systems without batteries are typically wired to provide from 235 V to 600 V, WSU reports. In battery-based systems, the trend is also toward use of higher array voltages, although many charge controllers still require lower voltages of 12 V, 24 V or 48 V to match the voltage of the battery string.
The inverters take care of power conditioning, converting the DC power coming from the PV modules or battery to AC power, and ensuring that the frequency of the AC output remains at 60 Hz. The inverter’s DC voltage input must match the nominal voltage of the solar array, usually 235 V to 600 V for systems without batteries ― and 12, 24, or 48 volts for battery-based systems.
The Maximum Power Point Tracking (MPPT) controller automatically adjusts the system voltage so that the PV array operates at its maximum power point. For battery-based systems, this feature has recently been incorporated into charge controllers.
Battery Bank Batteries store direct current electrical energy for later use. This energy storage comes at a cost, WSU reminds, since batteries reduce the efficiency and output of the PV system, typically by about 10% for lead-acid batteries.
In 2008, the installed cost of a residential solar energy system in the U.S. typically ranged from $8 to $10 per installed watt (before government or utility incentives). In 2009, the cost of modules in the United States ranged from $4 to $6 per watt. In a system without batteries, inverters are the second most costly component. In 2009, inverters cost on average less than $1 per watt. Batteries increase the complexity of the system, WSU concluded, and elevate the costs of both material and labor costs. In 2009, batteries cost on average $2 per watt-hour.
By Stephan Ohr, Consultant, Semiconductor Industry Analyst