With housing costs in densely-populated areas continuing to rise across the US, people are looking for more economical options. Additionally, about 48% of the energy used by homes in the US is for heating and cooling. Smaller living spaces equate to fewer resources - both for initial construction and over their lifetimes. In the long run, smaller spaces are more economical in terms of both energy, materials, and dollars. Small improvements in the efficiency of HVAC and energy storage systems could result in sizable carbon savings once scaled across the population.
The PUTT-PUTT will bring theory into practice, testing solutions to these problems on real human subjects (me) and against the inherent complications that come with implementing an idea - pure theory is often too neat. We want to bring comfort, sustainability, and flexibility to the bus.
The project therefore has two primary goals:
1) To design and build a modular and sustainable living space with the smallest carbon and spatial footprint possible. This living space will have all systems necessary for comfortable living: electricity from a solar-charged lithium-ion battery bank, plumbing, propane, and passive evaporative/radiant heating and cooling. The interior design will be comfortable and functional, with moving pieces of furniture that can adapt to any living situation. This will be constructed in the body of an old school bus, allowing for mobility.
2) To implement an experimental system for heating and cooling. The existing air conditioner infrastructure will be retrofitted with special cooling coils. An insulated water tank under the bus will store waste engine heat, which will be used to dry out the dehumidifying desiccant on which the cooling system operates. The idea is to implement and test an innovative system in a real-life situation to assess its viability for larger-scale use.
We also plan to use a Raspberry Pi and various auxiliary sensors to process data and evaluate resource use. We will record data about interior and exterior temperature and humidity, water use, electricity use, and number of people on board to assess the amount of resources used. Additionally, all the data and detailed construction information, along with a more informal blog, will be posted online in an effort to make this project open-source.
The Sustainable Energy System
For the same power, 24v systems are more efficient, both energy-wise and materials-wise, than 12v systems due to losses that scale with the square of amperage. Our solar system will therefore use a 24v, 1.4 kW solar array on the roof of the school bus to charge a recycled Tesla car battery pack rated for 5.3 kWh. An maximum-power-point tracking (MPPT) charger will moderate charging between the solar panels and the batteries and an inverter will be used to transform the 24v power into 120 VAC. Most solar storage systems use lead-acid batteries, but we hope to show that our system can be more efficient. The largest drawbacks to lead-acid systems include high lifetime costs and frequent maintenance. Lead-acid batteries should be cycled at most to a 60% depth of discharge for 500 cycles before its capacity decays significantly. Lithium-ion batteries, by contrast, are lighter, more efficient, relatively low on maintenance, and can be cycled over 1500 times to 90% depth of discharge. Their longer life expectancy and higher energy capacity make them more cost-effective. However, one problem with a lithium-ion system is the prohibitive cost of adding a battery maintenance system (BMS). We hope to create an economic cell balancer and monitor performance using inexpensive materials and a Particle Photon wifi module. Battery state, rate of use, and amount of energy consumed will be logged and graphed over time by a Raspberry Pi to track the effectiveness of our innovative system.
The largest drawback of living in a smaller environment is the lack of versatility of that space. There isn’t room to sleep visitors, there aren’t enough rooms, and so on. Therefore, furniture items and rooms need to serve multiple purposes. For this reason, many of the furniture items on the converted bus will be dynamic. The two couches in front will be used in various configurations for storage, tables, or even an overflow sleeping area. There will also be a retracting table should six people want to eat together. To maximize floor space, one of the benches will be able to slide under the workbench, allowing room for a larger number of guests to gather casually. The workbench can fold down to provide more room in the kitchen. The bathroom and shower are separate so that two people can use them at the same time, but for extra privacy and space for changing, the two doors can slide out to expand the space when necessary. The bedroom in back will incorporate storage under the bed, and the wardrobe will double as a desk. A small amount of wraparound seating will be available in case occupants want to retire to a separate room. In this way, we hope that a smaller space can serve its occupants with the same functionality as a full-sized home.
Open Source Design
In the spirit of skoolie culture, we hope to keep our build as accessible to the public as possible, whether that be through interaction with us via The Blog, our stream of Live Data, or our Open Source design plans. We'll update you as frequently as possible about the progress of our work. Don't hesitate to contact us via email on the Contact Us page or connect with us on Facebook or Instagram.