Anthony led investments at a venture management firm called 21Ventures, which has provided seed, growth, and bridge capital to over 40 tech companies, mainly in the cleantech space. As Fortune’s Dan Primack explains it, 21Ventures didn’t raise a traditional venture fund, but would identify startup investments and bring them to high-net-worth individuals or family offices for investing.

Anthony worked closely with investor David Gelbaum’s Quercus Trust, which back in 2008 was the third-most active venture fund investing in cleantech according to the Cleantech Group. However Gelbaum seemed to have started to pull back on cleantech investments in late 2009, and told me in a rare interview in February 2010 that he had lost money in cleantech and hadn’t had any exits then (though back then he was still optimistic).

Anthony wrote a variety of columns for GigaOM over the years, and I’ve interviewed him several times about his investments starting back in 2009. The last time I heard from Anthony was last September 2011, when he was working with home solar startup OneRoof Energy, which just recently raised more funding.

Here’s the last three articles Anthony wrote for us and an interview I did with him:

• A new energy storage option: gravity power
• The energy storage story
• Everybody loves clean energy but no one wants to pay for it
• And an interview I did with Anthony in the Summer of 2009: How to win at cleantech investing: milk stimulus, raid labs, befriend David Gelbaum

It’s not a secret that cleantech has proven to be more difficult to invest in than many had anticipated. Particularly when investors compare it to the web ecosystem and exits like Monday morning’s $1 billion acquisition of Instagram by Facebook. Part of the problem with cleantech investing is the long timetables for a lot of the startup investments.

Regardless of whether 21Ventures’ cleantech portfolio has had successes or exits, I think it’s important to list out some of the investments, so we can look at the collective amount of innovation that was under development by 21Ventures led funds. Anthony was passionate about cleantech innovation, as well fighting climate change and supporting environmental causes.

Anthony received his MBA from The Tuck School of Business at Dartmouth College in 1989 and a BA in economics from George Washington University in 1982.

Some of 21Ventures supported startups:

AeroFarms: AeroFarms is a New York-based startup that develops tech-heavy urban farming processes. Anthony told me he thought that advanced farming techniques were an under-invested area where his firm sees promise. AeroFarms presented at the WSJ Eco:nomics conference recently and was voted the #1 company that investors would want to put $1 million in.

OneRoof Energy: OneRoof Energy develops and owns home solar rooftop installations and recently closed more funding.

Gravity Power: Gravity Power makes a gravity power module that uses a very large piston that is suspended in a deep, water-filled shaft and a return pipe connecting to a pump-turbine at ground level. As the piston drops, it forces water down the storage shaft, up the return pipe and through the turbine, and spins a motor/generator to produce electricity. To store energy, grid power drives the motor/generator in reverse, spinning the pump to force water down the return pipe and into the shaft, lifting the piston. The company just created a Germany-based subsidiary. (Bill Gates and Bill Gross recently invested in a gravity energy storage play).

Magenn Power: Magenn Power makes high-altitude wind turbines. I haven’t heard about this company in years.

ETV Motors: An Israeli lithium ion battery cell startup founded in 2008.

Graphene Energy: Graphene Energy is an Austin, Texas, based-startup that has been developing a technology using graphene, which can produce at least twice the storage capacity of commercially available ultracapacitors. Ultracapacitors have ultra-fast charge and discharge times, but lag far behind batteries in terms of the amount of energy they can store. Graphene Energy was an ARPA-E finalist, but I haven’t heard from these guys in awhile.

Axion Power International: Axion Power has developed a lead-acid/carbon energy storage tech that blends ultracapacitor technology with old-fashioned lead-acid batteries. The problem with ultracapacitors for electric vehicles has been low energy density; next to batteries, the amount of power ultracapacitors can store per kilogram doesn’t measure up. But they do excel in charge time and lifespan. Ultracapacitors can handle far more charge cycles than lithium-ion or lead-acid batteries, and they recharge quickly. The company is public on the OTC bulletin board and not profitable.

GreenRay Solar: GreenRay makes a solar microinverter, which places an inverter on each panel, instead of the more traditional central inverter set up. The market for microinverters is growing, and Enphase Energy recently went public.

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Ski lift storage has been one of those topics discussed in the hallways of energy storage conferences for years. Someone was out there, the conversation went, that wanted to use a solar- or wind-powered ski lift apparatus as a pump. It would pump gravel or water up a hill during sunny periods. At night or during peak power emergencies, the gravel or water could be released. The system essentially artificially supplies the elevation that nature left out.

Such a system could even harvest regenerative power on the way down. It’s part of a segment I call macro or terrestrial storage, i.e. large mechanical devices that store power through the power of geography and gravity. Bright Energy Storage Technologies, for instance, wants to put giant plastic bags shaped like sea cucumbers connected to air hoses in the ocean. Seawater will contain and pressurize the water for free. It sounds strange, but the device could deliver power for 2.5 to 6 cents a kilowatt-hour, say the backers of the technology.

Research and project management company Escovale Consultancy Services talks about using a 100 million ton stone in a cavern to pressurize water,  similar to an idea being pursued by Gravity Power.  Think Stonehenge for the grid.

Then there are the micro-macro storage ideas: SustainX (compressing air in large tanks with water vapor), Isentropic (big tanks of hot gravel) and LightSail Energy (founded by an entrepreneur who entered college at age twelve.)

I scribbled down the name of the ski lift guys once at the Energy Storage Association conference in 2011, but I subsequently needed a paper towel and lost it forever.

Energy Cache

So who is that company? I am going to guess it is Energy Cache. Energy Cache wants to create a solar-powered pump for delivering materials to the top of mountains that can be released to produce energy. Here is one of their patents — note that one of the inventors is Bill Gross, the energetic dervish behind Idealab.

Claremont Creek Ventures is an investor and Idealab lists the company on its site. Gates himself wrote about the company in 2011 on his own blog.

Here’s a video of how the system works:

It’s an interesting idea, but one with many caveats. Terrestrial storage isn’t cheap. Compressed air energy storage, pumping megawatts of air into underground caves, has been around since the 1970s, but only a few trial systems have been built. Big projects take a lot of cash, and almost anything — geological surprises, changing world economics — can go wrong in the ten to fifteen years it can take to complete a project.

Environmental review can bottle up projects for years. Gridflex Energy has talked about building multi-megawatt hydro storage systems in Hawaii — the ocean would act as a free reservoir — and large systems in Montana. But good luck getting through the review process.

Some of the energy ideas from Idealab have stalled on the way to commercialization. Energy Innovations, a startup building concentrated solar photovoltaic technology, has reportedly put itself up for sale after struggling. Idealab also invested in Aptera, the three-wheeled electric car. The lightweight material Aptera used to build its car was fantastic — the car didn’t do so well.

Image courtesy of Andrew Miesem, Flickr creative commons and Energy Cache.

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Batteries are not environmentally or financially the best solution for grid-scale storage. Pumped Storage Hydro (PSH) — the only GW-scale storage technology deployed — and Compressed Air Energy Storage (CAES), with only two plants in operation globally, have given good results. But the construction of these energy storage options is costly, time consuming and wrought with environmental concerns.

So what’s the alternative? The answer may actually lie in digging deep rather than building up.

Pumped storage hydro uses gravity to store energy that is sourced from the grid by raising water to a higher altitude, creating potential energy. That potential is then converted to electricity when the water returns to its original level, passing through a turbine on the way. Storage capacity can be increased by adding mass and/or the storage height. An ideal site for pumped storage hydro would provide:

• a large elevation difference between two reservoirs (hundreds of meters or more);
• high power potential (1000 megawatts or more);
• large energy storage capacity (4 hours or more at rated power);
• negligible adverse environmental impact;
• proximity to power transmission lines and a major electricity market, such as a city.

Unfortunately, such ideal sites for pumped storage hydro do not exist.

However, a new technology now being developed exploits widely available analogous sites, using the proven technological components of pumped storage hydro in a completely new way.

Gravity Power Module

The figure above illustrates the basic design of the “Gravity Power Module” or GPM, which is being developed by 21Ventures portfolio company Gravity Power. Full Disclosure: As is the case with any venture capitalist like myself, there is some self-serving message here.

The GPM uses a very large piston that is suspended in a deep, water-filled shaft, with sliding seals to prevent leakage around the piston and a return pipe connecting to a pump-turbine at ground level. The piston is comprised of pancakes made from concrete and iron ore for high density and low cost. The shaft is filled with water once, at the start of operations, but is then sealed and no additional water is required.

As the piston drops, it forces water down the storage shaft, up the return pipe and through the turbine, and spins a motor/generator to produce electricity. To store energy, grid power drives the motor/generator in reverse, spinning the pump to force water down the return pipe and into the shaft, lifting the piston. Hundreds of megawatt-hours per shaft can be stored with high efficiency, since pump-turbines have low losses and friction is negligible at modest piston speeds.

Tackling the cost issue, economic operation of the GPM system depends heavily on the construction cost of the shaft, which is surprisingly low. This is because the GPM system will require less excavation per storage capacity than many existing pumped storage hydro facilities and because that excavation can be automated. A small footprint and unobtrusive operation will allow multi-shaft installations to be constructed even in dense urban areas.

Advantages include: modularity; use of existing technology; environmental compatibility; flexible siting; fast permitting; rapid construction; low cost per megawatt-hour; long lifetime; high efficiency; and a short time from project start to revenue.

The pump-turbine is capable of ramping from zero to full power in less than twenty seconds and has a broad power range, making GPMs technically superior to gas turbine power plants for ancillary services such as frequency regulation. Larger GPMs built in arrays can replace gas turbine peaking plants, providing a substantially lower levelized cost of energy (LCOE), and can replace intermediate power plants at comparable LCOE. The general parameters of two GPM installation types are listed below.

To really satisfy the world’s growing utility-scale energy storage needs, a technology must:

1. Provide hundreds of megawatts for several hours, per installation, with the dynamic operating characteristics required by the grid. Many storage technologies could do this, in theory. So far, only PSH and Compressed Air Energy Storage (CAES) have.
2. Achieve a competitive cost. Again, only PSH and CAES have met this goal, and few other technologies appear likely to.
3. Be deployable on a truly gargantuan scale. The International Energy Agency, in its 2008 Baseline scenario, estimated a worldwide need for over 250 GW of new coal-fired and gas-fired power plant capacity, per year, from 2005-2050. Avoiding an environmental catastrophe will require replacement of much of this with renewable generation and storage. Many current storage technologies will have beneficial roles to play, but none of them can achieve this goal.

The GPM can achieve all three.

GPM construction is less complicated than conventional power plants and uses commodity materials with local labor, making them suitable for fast, wide deployment in both developed and developing countries. Market penetration rates will be constrained only by the availability of trained construction crews and project financing. No new equipment factories will be needed for at least the first decade of deployment.

The availability of massive electricity storage will free renewable generation from one of its most challenging constraints—variability.  And because that storage can be constructed in place of conventional power plants, the incremental cost will be very small, perhaps even nonexistent. Wind and solar can provide the energy to fuel the world economy, and advances in energy storage capabilities such as the gravity power module will help.

David Anthony is the Managing Partner of 21Ventures, LLC, a VC management firm that has provided seed, growth, and bridge capital to over 40 technology ventures across the globe mainly in the cleantech arena. David Anthony is also Adjunct Professor at the New York Academy of Sciences (NYAS) and the NYU Stern School of business where he began teaching technology entrepreneurship in 2009.

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