In modern industrial society there is a major difference in urban life than in most previous urban societies. I currently live in an apartment in the city and I have a kitchen in which I cook, in most previous societies and even in many non-western industrialized societies I wouldn't have a kitchen and likely wouldn't even cook at all. If I lived in a city I’d have gotten cooked food in the street, whether from street vendors (think Malaysian hawkers or hot dog stands), restaurants, cafes and such. Thermopolium were the Roman equivalent of fast food and taverns could also function in a similar manner as well as being standard restaurants and the first drive through (for travelers) is thought to have been built in ancient Persia (Iran).
This is the ancient equivalent of fast food though it existed in a different context and plays a slightly different role in society. It was more nutritious, often contained less meat and was considered healthier and more important than contemporary fast food for example, and was designed to be served to a large group. A giant pot of soup mix, Paella or a mixture of curries for example, every urban culture has some equivalent. Street vendors also existed and they sold a variety of foods, Malaysian hawkers are on of the many surviving examples but their exist others. The criteria is that it is pre-cooked and is cooked in large batches.
The economic reasons for this are fairly simple, kitchens are expensive and in most cities were unaffordable for all but the well off. By having communal kitchens, you save on equipment and benefit from both specialization and the economics of scale that the individual private kitchen simply doesn’t provide. You can also save on fuel costs, something which is extremely important when fuel is both scarce and expensive (as it’s likely to be in the future). In the cities (and most towns) of the future, this model of cooking is likely to become near universal once again. The measure of its economic viability is the fact that for every 60 citizens of Pompeii there was 1 Thermopolium, and there existed other fast food dispensers, such as Popinas (wine bar, often for breakfasts such as wine soaked bread or vegetable stews) alongside more restaurant like places such as tavernas.
So this presents us with a problem in need of a solution. How do we take advantage of this model of cooking while fueling it with renewable energy? After all, most renewable energy sources produce mechanical energy (wind and hydro for example), while the main historical source of heat (biomass) is likely to be scarce and large scale use of wood in the cities would lead to deforestation. Where can the energy for this cooking model come from and how could it be put to use in a sustainable manner?
Overview of a solution:
The specific technical solution I’ll be looking at in this paper is that of combining solar concentrating technology with that of molten salt thermal storage to counteract the main disadvantages of solar cooking. The components of this solution are the; solar concentrators, the various transfers of the heat, molten salt storage units, the specialized cooking equipment needed and the difference between building integrated units and standalone units. Adapting this solution for mobile stands is also a topic and in this case is simple change in how the systems used. Also covered is the side project of producing a standardized set of connectors (similar to USB ports) between the various components to allow both a modular design and ease of distribution/production. Also, its possible to run refrigeration off of heat sources, so this system could easily be coupled with an absorption refrigeration system that uses the waste heat.
Specifics of the solution:
Solar Concentrators: How the salt is heat, the two methods are to either run the molten salt through pipes that are heated or focus the sunlight on a tower that contains the molten salt. For this application, solar troughs on the roof are probably the best bet and they would heat a central pipe through which the molten salt is pumped or flows due to heat differences. In some instances, such as buildings surrounded by open land, heliostats and a receiver tower could be the best option.
Molten salt thermal energy storage units: As a thermal energy storage method, molten salt has been around in various solar projects for 20 years and is also used in some non-solar industries as a heat transfer fluid. The system works by storing the salt as a liquid at 288°C in an insulated cool tank, then the salt is pumped into the solar collectors and heated to about 570°C. After that it is sent to the insulated hot storage tank where it can be stored for a week or so before the heat is used and the salt sent to the cool tank. Heavy insulation should be used so the slat doesn't freeze, through this shouldn't be a problem in the locations for which this solution is applicable.
There are two main storage methods to be considered. A single large tank as is currently used in stationary applications would be adequate for cafes, restaurants and other such eateries. The heat can then be piped to the cooking equipment as needed. The other option is to develop a heat battery that can be placed in vendor carts and such to power them for about a day, the batteries should not be to heavy but more research is needed to confirm, or delivered to small eateries unable to gather the heat themselves. The second type would simply be placed under a Fresnel lens until charged and each vendor could easily store 2-5 days’ worth of batteries (at home, not with the cart) at any time to account for intermittency. The other main design issue is how to allow a backup heat source to be used (say biogas) to heat up the salt when required.
Heat Transfer system: This is the component that transfers the latent heat stored in the salt tanks to the kitchen. The transfer of salt from the solar collectors to the tank is covered in that system separately, this is from the hot salt tank to the kitchen. For this specific model, the heat transfer will use the molten salt to create steam; the salt is then pumped to the cool tank and the steam to the kitchen pipes. Other such methods exist, through this method allows secondary use of the steam, such as steam cleaning, sterilization (say by an autoclave) or space heating. If steam heat grids are locally available, then this system's waste steam can be feed into it for further use.
Specialized cooking equipment: Since the heat energy is delivered via steam (or another such method), the cooking equipment will need to be specially designed for this system. A simple oven design would be to have an inner surface surrounded by a cavity in which steam can be pumped into which is then surrounded by heavy insulation. A fan could be powered (for the appropriate dishes) easily by a thermoelectric unit built into the oven, similar to the BioLite stove's system. Stove tops would have a cavity underneath them in which high pressure steam (to increase the temperature) can be pumped as needed, similar to today's electric stoves. Other devices could be modified as needed and using devices like hayboxes would certainly make the stored heat last longer.
Building Integrated units: If a restaurant, café or tavern is being built with the express purpose of serving a large quantity of food, then this system could benefit by being designed into the building itself. The pipes could be easily placed inside the walls and benefit from their insulation properties, the roof designed for solar collection and so on. The more challenging task would be to retrofit existing structures with this system due to the number of pipes required.
Mobile vendors: While they are currently far more common in Asia than in the West, street vendors were common in western pre-industrial cities. Malaysian hawkers, hot dog stands and small mobile cafes (like doughnut trucks) are all covered in this category. Here the use of small molten salt heat batteries would probably be the most useful, if enough heat can be stored then they’d simply be swapped in and out each day (or twice a day). The batteries could be charged by a Fresnel lens or other solar concentrator and stored in a specialized container. If both the container and charger could be the same equipment it would certainly save on costs. Weight is the main issue but as previously mentioned more research on my part will be needed to determine how large it potentially is. Their could easily be a charging station vendors visit often or a service to deliver charged batteries, it wouldn't be hard to time the system to function within the daily rhythms of life.
Standardization: If before large scale manufacturing began, the connectors and links between the various components could be standardized (especially for the mobile vendors), then a great deal of effort and resources could be saved. Especially if standardization of the salt batteries was done, similar to how batteries are classified now. If that was done, then the molten salt batteries could easily be used as a backup heat source for the buildings. This would allow components to be swapped in and out by the various end users according to their needs while also making the work of local manufacturers and repairmen much easier and simpler.
Absorption Refrigerator unit: Before electrically powered fridges came into use, absorption refrigeration was the standard. Seeing as it only needs a heat source to be powered and this system would be able to provide both significant primary and waste heat, this could easily be added as a plug and play component. Adding this component allows the system to be self-sufficient in all its core energy requirements (heating and cooling) as well as allowing the use of refrigeration something not able to done by by pre-modern eateries.
This is one way to potentially power a feature of urban life that is very likely to become near universal once again. The underlying technologies are all in use today and more solar cookers are being developed that use latent heat in some form, very little in the way needs to be invented for this system to work. All it requires is for a prototype to be built of the various models and a codified set of standardizations. There are two levels this project can be applied at, that of an individual to power an isolated vendor and that of a community (or similar scale) to build a sustainably powered restaurant, various government organisations would also find this project useful, such as militaries or prisons.