Proceedings of the International Association for Shell and Spatial Structures (IASS) Symposium 2013

„BEYOND THE LIMITS OF MAN” 23-27 September, Wroclaw University of Technology, Poland

J.B. Obrębski and R. Tarczewski (eds.)

The Solar Mountains

Raphaël Menard1, Etienne Fayette2, Paul Azzopardi3

1Director (Arch., Dpl. Ing.), Egis Concept // Elioth, Montreuil, France, r.menard@elioth.fr 2 Dpl. Ing, Egis Concept // Elioth, Montreuil, France, e.fayette@elioth.fr

3 Dpl. Ing, Egis Concept // Elioth, Montreuil, France, p.azzopardi@elioth.fr

Summary: The paper presents a variation of the solar updraft power: the solar mountain, fitted for rural, mountainous and sunny areas. The potential of this specific situation has been explored to propose a design, and then develop the technical and economic studies which confirm the feasibility.

Keywords: solar updraft tower, rural electrification, ETFE, dual-power strategy, tensile structure, greenhouse

1. THE SOLAR MOUNTAIN AT A GLANCE

The energy challenges of the next decades will not be overcome by widespread standardised solutions. Ignoring each site specificity has led to leave aside vast potential energy sources. The ecological and economic cost of concentrating and transporting energy to meet spikes in demand becomes less and less sustainable. At the same time, it appears that energy efficiency relies as much on technology as on user sensitiveness. All these elements push forward the overall idea to re-localize energy to both avoid expensive distribution costs and associate the energy sources to the development of local communities.

In this context, rural and mountainous areas are the frontier where economic development is restrained by the cost of energy. This situation may only get worse as the barrel price rises, broadening the gap between urban and rural standards of living and development potential. Meanwhile, many of these developing areas are bathed with sun: the Solar Mountains are the simple idea to turn a natural setting into a clean and reliable source of energy, servicing local development.

The principle is easy to figure out: a large scale greenhouse located at the foot of the mountain warms up vast quantities of air, which subsequently rises in a high chimney leaning on the mountain slope. The air flow is accelerated in the chimney, fast enough to drive turbines and produce electricity (see Fig. 1).

Fig.1: Conceptual view of the Solar Mountains

Based on basic and proven technologies such as greenhouses and turbines, the Solar Mountains are a way to provide electricity night and day at a local scale. The facility is easy to erect and to maintain, making it suitable to virtually any situation. It produces no waste or pollution. The impact on the environment and the land occupation is minimal as the Solar Mountains may be implemented in addition to several other activities: agriculture, biomass crop, photovoltaic production, emergency shelters…(see Fig. 2)

Fig.2: Inner view of the Solar Mountains

The Solar Mountains project is also innovative by its modesty: when humanity uncovers a clean and efficient energy source, Solar Mountains will be dismantled easily, will not leave scars on the landscapes thanks to their construction method and will be easily recycled.

2. PRESENTATION OF THE CONCEPT

2.1. Reference technology: the solar updraft tower

The Solar Mountains builds on the insights collected on solar updraft towers.

This technology uses the same physical principle as the Solar Mountain: a large greenhouse, enables to warm up vast quantities of air. The warm air tends to rise in the hollow tower and triggers a continuous and accelerated air flow, which propels a wind turbine located at the basis of the tower.

The concept of solar towers was first proposed in 1903 by Spanish Coronel Isidoro Cabanyes and the first representations of this technology were made by Hanns Günther in 1931.

The engineering office SBP (Pr. Jörg Schlaich’s consulting company) conceived in 1982 a 200 m-height prototype in Manzanares, in Andalusia, Spain with a nominal power of 50kWe. It comprised a collection area (greenhouse) of 46,000 m² (about 11 acres).

This pilot power plant operated for approximately eight years and was decommissioned in 1989 (see Fig.3)

Fig.3: The Solar Updraft Tower in Manzanares, Spain (construction in 1982, SBP)

However, it helped determine that the true energy efficiency of this technology should be met by a 1 km-high tower. The solar conversion yield is indeed proportional with the height of the chimney.

A project currently under consideration provides for a 200 MW power plant in Australia with a 900 m to 1000 m-high stack. Other projects are in feasibility in Spain (Fuente El Fresno) and Namibia.

Building such a tower remains cost-intensive, time-consuming for chimney erection and hardly suitable in seismic area, in territories where hurricanes could occur, or places where such building capacities are not available.

2.2. The Solar Mountains principle: a simple idea

The idea behind the Solar Mountains is actually very simple. Why spend efforts building a high-rise tower while mountains already provide an elevated support? The concept of solar tower is therefore re-shaped in accordance with this favourable natural disposition to become the Solar Mountains. The greenhouse collector is located downhill and favourably exposed on the mountain slopes. The chimney bends to lean on the mountain side up to the highest accessible points (see Fig.4)

This new approach enables the chimney to reach higher altitude with less structural effort. The Solar Mountains are thus a more efficient and cheaper evolution of the solar updraft tower. These features make the Solar Mountains a perfectly suited source of energy for sunny mountain areas, usually hard to connect to the grid.

Fig.4: Early sketches of the concept

2.3. Respecting the environment

The Solar Mountains collector structure may be supported by soft-impact foundations. This synthesis enforces the modularity and removability of the Solar Mountains.

Fig.5: Gradually expansion of the greenhouse

All the structural elements are made of tensile elements and ETFE membranes. Installing or removing parts of the greenhouse does not necessitate trucks nor roads: the elements can be carried in backpacks. Materials are subjected to change of definition due to local availability and prices (see Fig.6).

Fig.6: Backpack transportation and construction example

2.4. Delivering large amounts of renewable and stable energy

Unlike many other solar-powered energy sources, the Solar Mountains are little sensitive to the day / night cycle (see Fig.7).

Day

Water bags capture sun rays

Night

Water bags release sun heat

Fig.7: The thermal storage as a buffer for electricity production

Indeed, the use of thermal tanks (rocks, water or sand bags), that participate to the structure itself, enables heat to be captured in the day time and released at night to smooth production and profit from continuous resource availability (see Fig. 8).

In this way, the Solar Mountains are solar-based electricity generators that natively include “clean natural batteries” to supply reliable energy.

Fig.8: Thermal mass as generator inertia

3. ENABLING USE COMBINATIONS

Thanks to the thermal tanks, the Solar Mountains are a reliable energy source 24 h a day. Still, the field under the glasshouse collector is practicable and the cover filtrates only partially the sun rays, which enables a dual-energy strategy.

If one wants to put the emphasis on energy production, additional photovoltaic fields can be laid under the greenhouse to increase daily production (see Fig.9).

Fig.9: Additional photovoltaic panels increase electricity production

Fast-growing crops for biomass production can be grown inside the greenhouse (see Fig.10).

Fig.10: Combining electricity production and biomass production

The Solar Mountains are also completely suitable for agriculture, indeed water supply is available (the surface of the greenhouse can also serve as a condensation and sky water collector) (see Fig.11).

Fig.11: Combining electricity production and farming

4. COMPARISON WITH TWO COMPETING

TECHNOLOGIES

Solar updraft tower and the Solar Mountains may be compared on technical grounds:

4.1. Ability to catch the sun

Fig.12: Computer model of annual solar income: comparison between a solar updraft tower (left) and a Solar Mountain (right)

For the same greenhouse surface, the annual solar income is 10% higher for a Solar Mountain than for a solar updraft tower: while solar updraft towers always shade part of their collector, this does not occur with the Solar Mountains solution.

Furthermore, the slope of the mountain provides optimal orientation to catch sun rays.

4.2. Chimney structure

The chimney of the solar tower rises vertically over the landscape, up to 1000 m. This position demands structural efforts to stand lateral wind forces or event seismic vibrations whereas the chimney of the Solar Mountains leans on the mountain side: it is almost unaffected by outside wind forces or earthquakes. Structurally speaking, it is hardly more than a rigid tube anchored to the ground.

Latest structural developments envisage light-weight tensile structure made of cables and ETFE membrane. This design is compatible with man-scale erection processes, cheaper transportation to remote locations, and leaves ridiculous impact on the power plant site in the long term, compared with most traditional power plants (hydro-power, nuclear, coal, …). This lightness also implies modularity and flexibility in the expansion of the power plant with time, by simply expanding the surface of the collector greenhouse.

4.3. Photovoltaic cells and their necessary counterpart: batteries

Photovoltaic cells present a major drawback: the electricity produced needs to be stored in batteries.

Fig.13: Scheme of a casual photovoltaic installation

For the same annual electricity production (ie kWh of electricity per year), the investment for a photovoltaic farm needs to take into account the cost of the night storage by batteries (+20 % at least). Moreover, this type of system is hindered by the lifespan of electrochemical storage and environmental impact (heavy metals).

4.4. Economic forecast

It is difficult to realize economic projections on the solar mountains at large. Local conditions such as materials available, labor costs and the possibility of connecting the Solar Mountain to a pre-existing local grid play a crucial role. Major trends are nonetheless observable:

- the cost of electricity produced by the Solar Mountains is interesting from the outset and rapidly falls under 0.15€/kWh with generators over 30 MW, which is a highly competitive option regarding rural mountainous areas (Fig.14) with the capacity to tune energy supply with energy demand thanks to thermal inertial.

- the investment remains much lower than that of a Solar Updraft Tower.

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0 50 100 150 200

Power capacity in MW

elec

trici

ty c

ost (

€/kW

h)

SOLAR TOWER (@1800kWh)

SOLAR TOWER (@2300kWh)

SOLAR MOUNTAIN (@1800kWh)

SOLAR MOUNTAIN (@2300kWh)

SOLAR MOUNTAIN 2000m DUCT (@2300kWh)

Fig.14: Cost of electricity for investment rate @6%

5. CONCLUSION

The Solar Mountains are an easy and sustainable way to produce energy, support biomass production and farming while having low investment costs.

The following development is focused on making the structure lighter, simpler and reproducible.

Indeed, the success of the Solar Mountains will be reached with the involvement of local actors and the use of open-source materials.

The calculation tools developed by Elioth are aimed to be widespread and profitable to anyone who tends to develop the Solar Mountains.