MAKING GOOD USE
OF EXCESS POWER
The term “power-to-heat” is increasingly gaining importance in the transition toward renewable energies. The principle is simple: Electric current is converted into heat with an efficiency of nearly 100%. Upon closer inspection, the supposed ecological sin of converting valuable electricity into heat definitely makes sense and opens up a lucrative new market in times of extremely fluctuating electricity prices – even extending into the negative range.
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Strong fluctuations in the power grid due to renewable energies
Renewable energies are the future of power generation in Germany. They are, however, also associated with strong fluctuations in the power grid. Wind and photovoltaic power stations supply varying amounts of power, depending on the weather conditions. In order to balance out these fluctuations, the power generated from renewable energies needs to be controlled on the one hand and even disconnected from the grid in some cases, while the burdens on consumers need to be more flexible on the other hand (demand response). A constant frequency of 50 Hertz can only be maintained with complex control mechanisms. German grid operators are required to maintain the grid frequency at a constant level of 50 Hertz and to equalize fluctuations caused by an excess or shortage of power.
To prevent excess power from going unused, it can be converted into heat through power-to-heat systems and fed into a heat grid.
Setup and functions
Power-to-heat systems can be used both in the low-temperature range as well as in the high-temperature range (steam).
Immersion heaters or heating elements are used predominantly in the decentralized low-temperature range, while electrode steam boilers are used in the high-temperature range. The generation of process steam of up to 30 bar is technically possible by means of an electrode steam boiler. The saturated steam generated in this fashion can be superheated to higher temperatures by means of a downstream electrode flow heater and hence also fulfil higher requirements with regard to the steam generation.
Steam generation is based on the electrical resistance of the boiler water, whose electric conductivity should be analogously low (adjusted to 60 μS/cm). Efficient water conditioning is mandatory to achieve this. The efficiency of the electrode steam boiler is dependent in large part on the available water quality and is generally considerably higher than the efficiency of a steam boiler, because no thermal losses caused by hot waste gas occur.
Heat accumulators are not part of the standard outfit of a power-to-heat system, but they can enable a more flexible use. For instance, in the event of demand for balancing power, power-to-heat systems with heat accumulators can participate in the secondary and minute reserve market, without also being required to feed the heat into the district heating network (e.g. in the summer).
Investment costs for electric boilers used predominantly in the district heating network amount to approximately 75-100 euros per kilowatt. Costs vary depending on the area of application and the existing infrastructure. In the high-temperature range, the system must meet stricter requirements, and investment costs may therefore be twice as high. They amount to approximately 100-200 euros per kilowatt. Large-scale systems are particularly lucrative; at current prices in the balancing market, they can be amortized as early as after three to five years. Smaller, decentralized systems are less profitable, because they are associated with higher specific costs, while the specific revenue is the same.
Annual maintenance costs amount to nearly 3% of total investment and are comparable to those of a natural gas boiler system. There are no traditional wear parts, but a power-to-heat system requires still two days of maintenance each year. The pumps, valves and water/steam/condensation systems etc. are subject to the same wear as a conventional steam boiler system.
With a central power-to-heat system, the heat converted by the power is fed into a supply grid. The district heating system of a municipal utility is a classic example of such a heat supply grid. With it, the heat generated is transferred to the consumer through an isolated pipe system.
The advantage of CHP plants, such as a biogas CHP unit combined with a power-to-heat system is that they work very efficiently together, and there is often a connection with the subsequent utilization of the heat. The control of the power-to-heat system achieves a continuous operation of the CHP system, which can thus provide full capacity.
the future of the balancing energy market
The purpose of the balancing energy market is to balance fluctuations in the power grid. These days, greater emphasis is placed on the use of renewable energies for the production of power. Even though wind and sun do not produce the same output throughout the year, the power supply in Germany can be maintained continuously. This is achieved with balancing energy, which comes into play exactly when there is an imbalance between the generation and consumption. The reserve evens out fluctuations within seconds with the “primary reserve”, within five minutes with the “secondary reserve” or within fifteen minutes with the “minute reserve”.
Balancing energy is categorized into “positive balancing energy” on the one hand and “negative balancing energy” on the other. With “positive balancing energy”, demand for power is higher than the production capacities, and for this reason, power needs to be fed into the grid quickly. In the case of “negative balancing energy”, the grid is balanced when demand is too low by removing power from the grid. The operator of a power station that is actively participating in the balancing energy market receives a fee for the provision of additional capacities (“positive balancing energy”) or for reducing the feed-in (“negative balancing energy”). The main area of application of a power-to-heat system is in the area of “negative balancing energy.”
In times of negative electricity rates, some conventional power stations cannot be disconnected from the market as their regulating ability is too slow (e.g. nuclear power stations).
The capacity of conventional power stations can be reduced using power-to-heat systems, which ultimately achieves a more efficient use of renewable energies and a reduction of CO2 emissions.
Curtailment in cases of regional grid overload
Excess production of power from renewable energy (RE) systems may result in grid overloads, which require monitoring by the grid operator. If this is the case, wind power plants are switched off during periods of strong wind, but the operators of these plants are still eligible for the subsidies. The otherwise curtailed power can be utilized with a power-to-heat system, resulting in economic benefits. This principle of grid relief is achieved by way of so-called feed-in management.
Curtailment in times of negative electricity rates at
the stock exchange
Electricity rates at the stock exchange can reach a negative amount in some situations. This happens when the supply exceeds the demand. If the power-to-heat operator purchases this electricity, it allows him to use excess supply in a meaningful fashion to produce heat at low cost. Thus, the ecologically clean power generates heat in a very efficient manner. However, the hours with negative electricity rates presently only account for a very low share in the course of a year. In 2014, negative rates were recorded during 64 hours, which was equivalent to a low percentage of 0.73% relative to the entire year. But overall, there has been an upward trend. In 2015, the share of negative energies has almost doubled, to a value of 1.4%.
- Low specific investment costs for large-scale plants (capacity in excess of 10 MW)
- Depending on the power mix, considerable reduction in CO2 emissions (every MWh of steam generated from regenerative power saves 0.22 t of CO2 compared to natural gas)
- Annual revenue through demand response / sale of balancing energy in TRL and SRL [tertiary and secondary operating reserve]
- Additional power for electrode steam boilers can be purchased without an additional grid demand charge by means of appropriate load management → ideal in case of a temporary imbalance in demand for power and steam
- Proven technology, has been used in Scandinavia for 30 years
- Ideal for explosion-proof areas → no combustion, no fire
- Ideal for very strict emission requirements → no waste gas
- Electrode steam boiler can be operated unsupervised for 72 hours
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