Aladin II - Micro Combined Heat and Power Plant (mCHP)
Recent energy policies smooth the way for future strategies that are partly based on various renewable energy sources, such as solar or wind. However, these sources are strongly dependent on environmental factors. In this context, decentralized power generation with combined heat-and-power (CHP) units becomes increasingly popular, as those units do not only maximize energy efficiency by providing heat and power as close as possible to the consuming household, but are also able to provide regulative power to the electricity distribution network. The research project “ALADIN II” deals with the development of a micro combined heat and power plant (mCHP) which is not only highly efficient and highly flexible, but emits near-zero pollutant emissions.
This project is a collaboration between external pageBucher AG Langenthal (Motorex), external pageHoval Aktiengesellschaft, external pagesa-charging solutions, and ETH Zürich (IDSC & LAV).
It is funded by the Swiss Federal Office of Energy (external pageSFOE) and the Swiss Competence Center for Energy Research - Efficient Technologies and Systems for Mobility (external pageSCCER-Mobility).
Why CHP?
In light of the energy transition, more and more renewable energies contribute to the electricity mix. Some of these have an intermittent availability, e.g. solar or wind. In case of local weather phenomena, this increases the risks of having power outages. Combined heat and power can address this issue. The energy generation is decentralized such that consumers produce their own thermal and electrical energy. This not only minimizes transport losses, i.e. increases the efficiency, but also support grid stabilization if needed. CHP plants exist in different scaling level and are therefore available for industrial as well as private consumers.
Why research on CHPs, if they are already on the market?
First, the return-on-investment increases with the CHP size. Investment costs for large CHPs are typically amortized after very few years. This is why only very few small CHPs (micro and mini class) are available on the market. Thus, increasing the economic benefit of a micro combined heat and power plant is one of the key research areas. The other one is, naturally, the minimization of the pollutant emissions. Whereas many commercial CHPs are designed to just meet the governmental emission regulations, this project is focused on demonstrating the potential for emission reduction with cost-effective hardware and software solutions.
Project Vision
- Near-zero emissions
- High total efficiency
- High operational flexibility
- Fast load uptake
Performance Targets
- 5-10 kW electric power and high electrical efficiency
- Near zero steady-state pollutant emissions
(< 10 mg/Nm3 for NOX, CO, Total Hydrocarbons (THC)) - Low cold-start pollutant emissions
- Compact design
- Low investment costs
- Low maintenance and long lifetime
Plant Design
The mCHP plant is powered by a 1-cylinder spark-ignited combustion engine with 325 ccm displacement volume. The engine has been adapted to run on natural gas. The exhaust gas flows through a three-way catalytic converter in order to remove the combustion residues. The engine always operates at full load. It drives an asynchronous generator that produces the electrical energy. This electrical energy can then either be used directly on site or, if there is no power demand, it can be fed to the electricity grid for a compensation according to the effective feed-in tariff.
Hot water is typically supplied at temperature levels around 80°C to the thermal energy storage (TES). This thermal energy is generated via two paths. First, the hot exhaust gases are routed through a shell-and-tube heat exchanger and, thus, heating up the water mass contained in the shell. Second, thermal energy is transfered from the engines cooling circuit via a plate heat exchanger. Thus, compared to an automotive context, the heat removed from the engine is not released as waste-heat to the environment, but utilized to produce the plants' thermal energy output.
Steady-state pollutant emissions
The use of a three-way catalytic converter (TWC) significantly reduces pollutant emissions. A cascaded control scheme of two feedback-controllers for the air/fuel ratio upstream and downstream of the TWC ensures steady-state operation with near-zero pollutant emissions.
Cold Start Emissions Reduction
One key objective in the design and operation of the mCHP is its flexibility. This means that the plant undergoes several ON/OFF cycles compared to conventional CHPs that, once switched on, operate until the heat storage reaches its capacity threshold. However, the prime mover is an internal combustion engine and from the automotive engineering literature, it is well known, that the pollutant emissions during a cold-start account for a large fraction of the total emissions. With just the steady-state control algorithms running, we would therefore dramatically increase the environmental footprint of the entire plant.
By developing simple and cost-effective control strategies that can run on any industrial ECU, we have been able to bring down transient emissions by up to -91% for nitrogen oxides (NOX). Reducing pollutant emissions typically poses a tradeoff between the relevant emissions species. We have put more weight on reducing NOX rather than carbon monoxide (CO), as conversion of CO inside the TWC happens at much lower temperatures and therefore more quickly than NOX. Furthermore, NOX regulations are more stringent.
In total, our transient emission reduction strategies are effective to such an extent that the average pollutant emissions for a 1-hour run with cold-start fall below the governmental regulations that are in effect for pure steady-state operation.
Efficiency
Maximizing the efficiency is a two-stage process. First, we focus on the system-level. Firing up a CHP triggers a warm-up process that takes a certain time. During this time, fuel is burned, i.e. energy is put into the system. The energy output, however, is just the electrical energy produced by the generator up until the time, when the system reaches its steady-state temperatures. Only then can heat energy be transfered to the heat storage. This means that CHPs have a very high efficiency during steady-state operation but a very low efficiency during transient operation.
Our research focusses on developing and applying intelligent control algorithms that accelerate the warm-up process of the plant to minimize the time of low total efficiency. As it is the case for pollutant emissions, this is crucial in order to focus on enhancing the plants flexibility and conduct research on the operational level.
Flexibility/Demand-driven operation
Second, after optimizing the warm-up process to maximize the efficiency, we will do research on the operational concept of such an mCHP. More specificially, we want find out if there is a certain threshold or range in terms of time-to-steady-state that triggers flexible operation to lead to economic benefit.