After two decades of flat demand, US electricity use is climbing and the race is on for generation sources that can operate around the clock and reliably deliver the power needed to run an increasingly energy-hungry US economy.

By Gavin Maguire Wind and solar are cheap but intermittent. Natural gas supplies are abundant but there are long delays for new pipelines and the turbines needed to burn it.

Coal plants face tough environmental restrictions and are hard to supply cheaply, while conventional nuclear power plants and hydro dams are hugely expensive and often have decade-long construction time lines.

That leaves a growing need for power sources that can deliver reliable electricity around the clock without the carbon footprint of fossil fuels, and explains the rising interest in next-generation geothermal systems, advanced nuclear reactors, floating solar arrays and other emerging technologies.

Here are six contenders, from the proven to the audacious, that are on the map of early investors in the energy space and could unlock the next wave of reliable, low-carbon power generation.

Each has been touted as a potential solution to surging electricity demand without deepening dependence on fossil fuels, but also faces key unknowns about how effective and affordable they will be if they reach full operational scale.

Fervo Energy's Cape Station site in Utah is the first greenfield enhanced geothermal plant in the world to hit commercial operation, and it got there early.

Fervo uses horizontal drilling and hydraulic fracturing techniques perfected by the oil and gas industry to create geothermal resources where they don't naturally exist.

Key unknown: Can drilling costs keep falling as Fervo moves to new sites, and can each site deliver the reliable power flows needed for highly demanding data center consumers? TVA's Clinch River, Tennessee, site received the first US construction permit for a commercial small modular reactor (SMR), and is the test case for whether factory-built parts can overcome the nuclear industry's long-standing problems with cost overruns and construction delays.

Key unknown: In theory, scalable nuclear reactors can deliver reliable clean power wherever it is needed.

But can developers perfect the modular off-site manufacturing, supply chains and construction methods needed to build reactors quickly and cheaply enough to compete with rapidly falling costs for renewable power, batteries and natural gas generation?

If they can, SMRs could emerge as a major source of new electricity supply.

The Gila River Indian Community in Arizona built the first solar-over-canal project in the US, which generates power and slows water evaporation at the same time.

Similar projects are attractive because they ensure that no farmland is lost to solar arrays while promising significant water savings in drought-prone areas.

Canal routes can also provide ready-made corridors for transmission and distribution infrastructure, potentially reducing development and permitting hurdles compared with greenfield solar projects.

Key unknown: The installation of solar arrays over bodies of water can cost substantially more than conventional systems, so can costs of deployment be steadily lowered to broaden their appeal?

TerraPower's Bill Gates-backed natrium reactor is being built beside an old coal plant in Kemmerer, Wyoming, and so can tap into already-existing grid connections once it is up and running.

The sodium-cooled reactor combines rampable nuclear generation with molten-salt storage, which on paper will allow it to increase output when electricity demand peaks and pare back generation during consumption lulls.

With several US coal plants facing end-of-life decommissioning decisions, the coal-to-nuclear narrative has broad social support and has strong job-creation potential in areas otherwise facing redundancy risks.

Key unknowns: Project completion is slated for 2030, which means it runs the risk of obsolescence if other projects get up and running first or if the complex first-of-its-kind Kemmerer undertaking runs into construction delays.

Kairos Power's Hermes 2 reactor in Tennessee has gleaned a lot of early attention as it already has a customer lined up to pay for its power (Google) and can run at lower pressure than conventional reactors thanks to molten salt pebble cooling.

It is also being built at Oak Ridge, which was the primary home for the Manhattan Project and so has almost unrivaled pedigree in terms of nuclear tinkering expertise.

Key unknowns: In addition to testing the modular construction theory facing other new reactor developers, the Hermes 2 plant needs specialized high-assay, low-enriched uranium (HALEU), which is still scarce in the US.

HALEU allows reactors to operate for longer between outages, and can deliver higher power density and use smaller reactor cores than older nuclear plants.

However, Russia and China remain the only major suppliers of HALEU, and US government-supported firms have only recently begun efforts to create domestic supplies which require the construction of conversion facilities, fabrication plants and transportation infrastructure that may take years.

Helion Energy's Orion is a planned 50-megawatt fusion power plant under construction in Chelan County, Washington.

Most fusion designs work by generating heat from nuclear reactions, which then boils water to drive turbines. Helion is trying something different.

Its reactors will create and compress plasmas in short pulses and aim to convert the resulting energy directly into electricity through magnetic fields, bypassing the conventional steam-turbine process.

If successful, the process could reduce plant complexity, improve energy-conversion rates and enable smaller reactors to deploy the process.

The fuel needed for fusion plants is also relatively abundant (mainly hydrogen isotopes), and several US research facilities have built up expertise in that fuel's generation and storage.

Key unknowns: Fusion has produced impressive experimental results for decades, but no company has yet demonstrated a commercial power plant that generates more electricity than it consumes.

Helion has also recently pushed back its initial estimated delivery timeline, which had been 2028, to potentially 2030 or later, which may mean Orion could arrive too late for some of the most demanding potential customers.

All told, these projects all offer potentially revolutionary breakthroughs in power generation, and could transform the US power system from one of scarcity to abundance that could supercharge the US economy in the following years.

Some are already selling power, and some may never. The rest will tell us by 2030. The opinions expressed here are those of the author, a columnist for Reuters.