As my colleague Mike Toman noted  recently, Geoffrey Heal of Columbia University said the following in a recent blog post :
"neither costs nor capital requirement will prevent us from decarbonising the electricity supply. The real obstacle to doing this largely with renewables is our current inability to store power, and as long as we cannot store power we will need to use non-renewable sources like nuclear and coal with carbon capture and storage."
However, this view does not factor in future technological innovation, which I think is very significant.
The IEA Energy Technology Perspective projected that renewable energy could contribute around 50% of the power mix by 2050 under their Blue Scenario to achieve a 450 ppm world. Many other global leading energy/climate scenarios have the same projections, including those from Shell. Of renewable energy resources, geothermal, hydro, and biomass can provide base-load power. Indeed, solar and wind are intermittent.
A large share of these intermittent resources in the grid system may affect reliability, but this can be addressed in a variety of ways -- through hydropower (including pumped storage), load management, energy storage facilities, interconnection with neighboring provinces and countries, and smart grid management. Renewable energy resources in a power portfolio, such as wind and hydro, can complement each other. Smart grids can enhance the reliability of electricity networks. Concentrated solar power, with melted salts storage, can also provide base-load power. For example, ESKOM in South Africa is developing such technology providing a 68% load factor, based on the US pilot.
In addition to technical barriers, regulatory and institutional barriers also need to be addressed, particularly in developing countries. For example, grid companies usually have no incentives and bear additional costs to purchase intermittent renewable energy. To overcome this barrier, governments can mandate the grid companies to purchase renewable energy and provide incentives to compensate their additional costs.
With the enabling supportive policies and technology innovations, the 50% renewable energy trajectory is within reach. Denmark has already achieved 20% power production from wind alone, and 30% from all renewable energy sources. It sets an ambitious target to increase its renewable energy share in power generation to 50%.