Hot Water

 

More than half of all the energy consumed in residential households is in the form of heat. We feel comfortable at room temperatures around 20°C and, of course, we use hot water for showers and bathing. One really might not think it, but we literally drain 1/6th of all the energy we consume into the sewerage when we wash. Now, what’s the best way for producing all this warm water?

 

Renewable heat sources

Renewable sources like sun are efficient warming up water and should be used, when possible. Unfortunately, in Europe, for example, during the winter time use of sunlight is just not an option for continuous heating. Thus, other sustainable solutions to ensure heating of our residences need to be sought.

 

Cogeneration of power and heat

We need electricity.

We need lighting and we need to power our mobile phones, computers, etc.

When electricity is being generated, there are always losses from the primary fuel and usually these losses occur in terms of waste heat, i.e. the exhaust. This waste heat is something one should not dismiss. The amount of energy in the exhaust stream usually surpasses the amount of electrical power being produced.

It is sustainable to recover this excess heat and use it for filling the heating needs described above. This kind of simultaneous power and heat generation is called “cogeneration” and power plants operating this way are “Combined Heat and Power” plants, or CHP-plants.

Energy consumption in homes in USA.

According to the UK Department of Energy and Climate Change, by generating heat and power simultaneously, CHP can reduce carbon emissions by up to 30% compared to the separate means of conventional generation via a boiler and power station.

 

Three essential dimensions in cogeneration

When the waste heat of power generation is being captured and used locally, the whole process is highly efficient, as losses are minimal. This all makes sense and may sound very simple, but the truth is a bit different; CHP generation can be very complex.

Firstly, unlike the electricity that is generated and can be distributed relatively efficiently over long distances, heat really can’t. The heat is usually distributed by a hot water piping system from the power plant to the place, where it is being consumed. The length of this pipeline system cannot be excessively long due to pumping and heat losses in the circuit. What’s more, the temperatures in the system need to match.

In other words, it makes no sense to circulate water of just 20°C in the radiator to heat up a room to a temperature of 20°C and, moreover, during the cold day, that room temperature would fall well below 20°C.

All the things described above are “passive features” of the heating loop. In other words, these can be fixed by a proper design of that heat loop. The active feature is the balance of using such a loop.

The heat and electrical power needs do not necessarily occur the same time. The heating needs to be turned on during the night time when only little electrical power is needed. Even in the winter-time, people moving in apartments and sunshine might warm up the buildings so that during the day time, when electrical power consumption is at its peak, not that much heat is needed. This non-simultaneous need of power and heat also appears in industrial solutions and needs to be considered when designing cogeneration plants.

In the two graphs below, different power vs heat patterns are being shown. The first is modelled after a commercial / office building energy use pattern and the second is a 3-shift factory manufacturing electrical demand type.

Site energy use - matching or close to matching heat and power loads.

 

Site energy use - non-matching heat and power loads.

In addition to the passive and active parts of the cogeneration described above, there is also third aspect to consider. This is related to the power plant itself; how recoverable is the excess heat and what happens if it can’t be recovered?

The usual way to recover excess heat is to place a heat exchanger to recover the excess heat from, say, exhaust gas stream to hot water loop. This would reduce the out-going exhaust gas temperature and increase the water temperature in the heating loop. The result is better economics for the plant owner, less CO2 is released to the environment and less primary fuel consumed overall.

In case the waste heat can’t be utilised, it should be dumped to the atmosphere, but this is not always the case and can make things more complicated. An example of this complexity is the cooling water of an engine. This water needs to be cooled all the time, regardless of whether the recoverable heat is being used or not. In order to have the engine operating optimally, the heat recovery of this loop is in parallel to the cooling system of the original loop, it should not “see” any difference in its own hydraulic circuit. The multiple sources of excess heat also mean more complexity and losses on the recovery processes.

Example of energy flows of a reciprocating engine.

 

Aurelia’s approach

Aurelia’s turbines are designed for flexibility and energy efficiency. Turbine technology inherently underlines these two features – we just want to make it work well at smaller scales too.

Our turbines have high electrical efficiency, by far highest in our class. This makes the plant operation profitable for our CHP owners even when the heat is not needed; and when it comes to the heat recovery, our turbines can be installed so that there is only one source of excess heat – the exhaust gas. This reduces the losses of the heat recovery process and improves overall efficiency.

Furthermore, the excess heat of our turbines can be released to the atmosphere if it is not required. This makes the engineering of the installation much easier and also reduces the costs of installation– not to mention the enhanced simplicity to match the plant output to real demand.

Principle of the Aurelia - CHP. Note the possibility for flexible operations.