Project Archimedes: A hybrid thermal solar collector for distributed power generation and water purification.
Project Description
Archimedes. (c. 287 BC – c. 212 BC) was an ancient Greek mathematician, physicist and engineer. Although little is known about his life, he is regarded as one of the most important scientists in classical antiquity. In addition to making important discoveries in the field of mathematics and geometry, he is credited with producing machines that were well ahead of their time. During the Second Punic War, Archimedes is said to have repelled an attack by Roman forces by using a "burning glass" to focus sunlight on the approaching ships, causing them to catch fire. This claim is sometimes referred to as the "Archimedes death ray" (source wikipedia: http://en.wikipedia.org/wiki/Archimedes )
While the thermal solar collector is being used for a much friendlier purpose than the "Archimedes death ray", it’s principles are the same, namely concentrating sunlight to a small spot to achieve very high temperatures.
This project is for a hybrid thermal solar collector suitable for distributed power generation. This means that the collector has to fit into a residential environment. Since the collector is of considerable size, this is a great challenge in the architectural area (see the architecture section).
The collector will be able to generate power and other services during day and night. This means that the collector has to store enough energy during the day to keep on working during the night and on overcast days. To store the energy, the collector uses a salt container (see molten salt container section).
The application should be able to work standalone (off grid) or connected to the grid. It is important that the device can handle both situations because it will give it a big head start over the other sustainable energy generation methods. The direct advantage of netzero power generation and water provision, is actually an advantage for the utility company, since they don’t have to install and maintain a connection to the premises only to fill in the gaps when the device does not deliver enough power and/or water. For rural areas the benefits are obvious. A general advantage of distributed power is that the distribution factor does not have to be taken in account. With utility power half to one third of the kWh price exists of distribution costs.
The molten salt container is able to store about 250-300 kWh of thermal energy. The generator should be able to deliver 6-10 kWe. This might seem as a lot for a normal household. However, this type of device will generally be used in sunbelt countries, where often the temperatures reach high values. Houses on these locations are generally equipped with air conditioners which are notorious energy consumers (Every small air conditioner can easily swallow 1 kWe). Furthermore, the application is built to last for a long time (at least 20 years). Though electrical devices are getting more and more efficient, it is expected that the general power consumption of a household will not go down, but up in the coming years. This design anticipates this.
The dimensions of the dish are 9 by 9 meters with an aperture area of 72m². Through a large surface, a lot of energy can be gathered in a short time. This will make the collector suitable for locations where there is less direct sunlight than a desert like area. More collectors can be placed in a cluster to provide power and water for small communities (residences).
The collector will be used in moderate climates where normally there are no excesive winds to be expected. In case of windy conditions, the collector will return into stow position. Preferably the collector will be build using lightweight materials like aluminium (see the Construction section).
Parabolic mirrors are being used to concentrate the solar energy into a receiver (see receiver section). Where the heat is transferred downwards through a stream of molten salt. The collector overall performance is strongly dependent on the quality of the mirrors, and the efficiency of the receiver and transportation process (see the mirrors section).
The Hybrid device consits of a fossil fuel (or hydrogen) burner to keep the molten salt container at the right temperature. Ideally it should not be used when the collector is correctly proportioned for the energy it should deliver. However wether conditions can be unpredictable and therefore it is a manatory part of the thermal collector (see hybrid device section).
A conversion unit is nessesary to convert the stored heat into electricity. This goal can be acceived in several ways. In this concept, a Stirling engine (hot gas) engine is used. However a microsized steam turbine will also be possible (see conversion unit section).
Does the device also work at night?
The energy collected during the day is stored in the form of heat in a molten salt tank at high temperatures. This way a big amount of energy can be stored in a relative small salt container. On sunny days this should be sufficient to generate enough power throughout the night. On overcast days the hybrid device will take over and provide the heat necessary to power the conversion unit.
Why the collector has to be so big?
One of the design goals is to make this solar collector “net zero”, which means that it should work without any connection to the grid. To make the collector also suitable for areas with moderate solar levels, the device has to capture more energy than in high solar areas. Furthermore it should use the hybrid device as little as possible. Therefore the collector is somewhat oversized in order to collect as much energy as possible, when there is direct sunlight at the location.
How much electricity, heat and fresh water will the device produce?
The average energy production for this design on a location with a insolation level between 5 and 6 kWt/m²/Day, should be 50 kWe and 150 kWt per day. The fresh water production depends on the type of purification device that is used. It is estimated to be 2-3 m³ per day. The maximum electrical load of the system must not exceed 10kWe.
Is this device safe?
A possible danger area is the molten salt container, containing salt at a temperature around 700 degrees Celsius. To minimize the risks there, the salt container is surrounded by a concrete layer and completely closed from unauthorized access. In case of a tank rupture the salt will leak into the concrete wrapper without any danger to persons or the environment.
Why are there not yet millions of these devices around the world?
Until now the development costs of thermal solar solutions were too high to make them competitive with other forms of alternative energy and traditional energy sources. Traditional development always result in some kind of prototype, which will do the job, but is not suitable for mass production. Through the “open source” concept we will lower the development costs and speed up the overall process at the same time. By putting the design in the public domain every interested company or organization can begin producing parts or complete systems resulting in more capacity, more standardization and lower production costs.
What will it cost to build a hybrid thermal solar collector?
When produced in greater numbers, the projected price of this type of solar collector will be less than € 20.000. excluding installation and additional add on devices (water purification, air conditioning, backup power, etc).
What is a external combustion engine?
An external combustion engine (EC engine) is a heat engine where an (internal) working fluid is heated by an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine produces motion and usable work. The fluid is then cooled, compressed and reused (closed cycle), or (less commonly) dumped, and cool fluid pulled in (open cycle air engine).
"External combustion" refers to burning fuel with an oxidizer, to supply the external heat. Although the engine configuration is the same whether the heat comes from combustion, or some other heat source.
The internal fluid can be a gas as in a Stirling engine, or steam as in a steam engine. The fluid can be of any composition; gas is by far the most common, although even single-phase liquid is sometimes used. In the case of the steam engine, the fluid changes phases between liquid and gas.
http://en.wikipedia.org/wiki/External_combustion
What is a Stirling engine?
The Stirling engine is a closed-cycle piston heat engine. The term "closed-cycle" means that the working gas is permanently contained within the cylinder, unlike the "open-cycle" internal combustion engine and some steam engines, which vent the working fluid to the atmosphere. The Stirling engine is traditionally classified as an external combustion engine, despite the fact that heat can be supplied by non-combusting sources such as solar and nuclear energy. A Stirling engine operates through the use of an external heat source and an external heat sink, each maintained within a limited temperature range, and having a sufficiently large temperature difference between them. (source wikipedia: http://en.wikipedia.org/wiki/Stirling_engine ).
What is the Rankine Cycle?
The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. Almost all coal and nuclear power stations use this cycle for power generation. It is named after William John Macquorn Rankine, a Scottish polymath.