As the number of people on our planet increases at a rapid rate the need for clean and reliable sources of energy is set to become pressing. The United Nations currently estimates the global population will reach 9.7 billion by 2050.

Ahead of CNBC's Sustainable Energy Brainstorm event taking place on July 9 in Paris, we take a look back at some of the clean energy technologies and ideas that Sustainable Energy has previously featured. These innovations are aimed at helping transform the lives of people around the world and securing their energy future.

The idea of generating electricity from the waste product of pigs and chickens might seem unappealing to some, but for others, it is a renewable energy opportunity.

In March, electric power holding company Duke Energy announced it would buy poultry and swine waste from a planned facility in North Carolina, using the methane produced to generate renewable electricity.

In May, Duke Energy announced a second deal to purchase "methane gas derived from swine waste."

Our sun is a huge source of clean energy, and in 2014 the International Energy Agency stated that the sun could be the planet's biggest source of electricity by 2050.

U.K company Renovagen is looking to harness the power of the sun - wherever it shines - with "rollable" solar arrays that can be rapidly deployed in a variety of locations.

John Hingley, Renovagen's founder and CEO, told CNBC in March that the company had designed a rollable solar farm, with the power cabling and support structure embedded into a "solar mat".

An 18 kilowatt unit can be deployed via a trailer in roughly two minutes; a larger system of as much as 300 kilowatts in size can be operational in less than an hour.

Potential areas for the application of the technology include disaster relief, humanitarian response and the military.

"The end goal for me is the day that I see our product save a life. It will be all worthwhile … regardless of what else happens," Hingley said at the time.

Staying on the theme of solar, could the key to a clean energy future be found in the heavens?

NextPV – an international joint laboratory between France's National Center for Scientific Research (CNRS) and the University of Tokyo – has come up with the idea of harnessing solar energy by placing solar panels on balloons and sending them above the clouds.

"Balloons high in the air can harvest much more energy – five times more – in a very predictable way. This is also a solution available everywhere on the planet," Jean-Francois Guillemoles, senior researcher at the CNRS and visiting professor at the University of Tokyo, told CNBC in January.

Guillemoles went on to add that the project was investigating the potential of tethered balloons at around 6 kilometers in altitude.

The roads and pavements we drive and walk on could be huge untapped sources of clean energy.

The SolaRoad – the idea of bright minds in the Netherlands – is made from pre-fabricated concrete slabs, 2.5 by 3.5 meters in size. The idea is that radiant heat and light are captured through solar panels.

The SolaRoad project was developed by Dutch research institute TNO, the Province of Noord-Holland and road construction firm Ooms Civiel, among others.

According to SolaRoad, a translucent layer of tempered glass – roughly one centimeter thick – sits on top of the slab, with "crystalline silicon solar cells" below the glass. SolaRoad says that the top layer has also been designed to be dirt repellent, skid resistant and strong.

"The idea we started with was 'wouldn't it be fantastic if we could harvest the solar energy that hits our road network, convert it to electricity and then perhaps even power the cars that drive over the roads?'," Sten de Wit, spokesman for the SolaRoad consortium, told CNBC at the end of last year.

In the developed world, many of us take clean drinking water for granted, but millions of people face a real struggle to get access to a clean, safe supply.

In Oregon one company, Puralytics, has developed a system that uses photochemical processes to break down and get rid of contaminants from water.

"It uses the light – of either the sun or of LEDs – to activate a nanotechnology mesh which causes five processes at the surface of this mesh," Mark Owen, founder and chief executive of Puralytics, told CNBC back in November.

"Some of them draw the contaminants into the mesh and then they break apart the contaminants at the molecular level," Owen added.

The technology developed by Puralytics has already been used in disaster zones to treat water for affected communities.

Transitioning from gas guzzling vehicles to ones running on cleaner sources of fuel will be hugely important if we are to move to a greener, more sustainable planet.

A range of ideas are being formed, as bright minds look to make clean transport viable. These include solar powered cars and planes, and buses fueled by human waste.

The potential of clean transport is clear. At the end of June, Solar Impulse 2, the solar powered airplane, completed a mammoth 70-hour flight across the Atlantic from New York City to Seville, Spain.

"The Atlantic has always been this symbol of going from the old world to the new world, and everybody has tried to cross the Atlantic with sail boats, steam boats, airships, airplanes, balloons, even rowing boats," pilot Bertrand Piccard said in a speech after landing in Seville.

"Today, it's a solar-powered airplane for the first time ever, flying electric, with no fuel and no pollution," Piccard added.

For many people around the world – especially those in rural areas – living off the grid is part of normal life. This presents a series of challenges, including intermittent electricity supplies that can hamper peoples' lives in many ways.

Now, companies are looking to use the deep penetration of mobile technology in Africa to introduce the idea of "pay-as-you-go" solar power.

Cambridge, U.K., based Azuri Technologies has devised one such system, where end users in rural communities pay a one-off fee for a solar power system's installations, and then use their mobile to top that system up, buying credit on a monthly or weekly basis.

In Kenya, M-KOPA Solar is also using a "pay-as-you-go" model, and says it has connected over 330,000 homes in Kenya, Uganda and Tanzania to solar power systems.

"We believe we've developed a proposition which has global potential … [but] our core focus is east Africa and we're still really just getting started here," Jesse Moore, co-founder and CEO of the company, told CNBC back in 2015.