Combining Heat & Power

Wind turbines and solar cells generate electricity. Electricity can also be produced by solar panels that - like magnifying glasses - concentrate sunlight to high temperatures. This article compares use-cases and their consequent economics.

Wind turbines may not be a significant part of the energy mix in the future. As much as everybody wants wind turbines to produce electricity for all of us, nobody wants them in their backyard nor disturbing their undisturbed view over the sea.

Looking at energy output the fundamental difference between wind turbines and solar cells (also known as photovoltaics or simply PV) on the one side and solar thermal on the other side is that solar thermal generates heat which can be used for both heat and electricity purposes whereas wind turbines and PV only produce electricity. Generating and utilizing both heat and electricity at the same time lowers the total cost of energy.

As wind turbines and PV produce electricity they potentially address close to 20% of the present global energy consumption.

Presently the contribution of wind and PV to the total final energy consumption is not much more than 1.35% (7.9% of 17%).

With annually $300 billion in investments in new capacity, investments in non-renewables are at the same level as the total investments in PV and wind, with fossil fuels enjoying an additional $300 billion in subsidies. My guess is that it will take many years for this picture to change dramatically.

A recent study finds that in a very-best-case scenario where the world reaches CO2-neutrality by 2050 and all possible processes have been electrified, heat-driven processes will still demand the major load of our global energy consumption.

An estimated 75% of global heat consumption is for processes below 400C, which means that this heat can be delivered by existing solar thermal systems. A major reason this hasn’t happened yet is that their costs until very recently haven’t been able to compete with fossil fuels. For further insights into methods of heat generation, please refer to my “Generation” article.

Heat drives electricity production for all other methods than PV and wind- and hydro turbines. When electricity is produced from heat – as in all fossil-fired power plants – then most of the energy is returned as waste heat. The amount of waste depends on the specific set-up. The best power plants convert 62% of the produced heat to electricity but, as a rule-of-thumb, roughly 1/3 is turned into electricity and 2/3 returned as waste heat. The higher the conversion efficiency, the higher the cost of the equipment. And vice versa.

Waste heat gives energy to district heating and many desalination plants around the world. Hence, systems with combined heat and electricity produce 3 times as much useful energy as systems that produce just electricity. Thereby, they also reduce the average cost of the produced energy significantly.

When comparing wind and PV with fossil-fired electricity this is important to remember. E.g. when Vattenfall claims that an offshore wind farm on the Danish west coast producing power at €64/MWh is competitive to market prices, then this holds true only when compared to fossil-fired production where the waste heat is not utilized. The reason: If natural gas costs €32/MWh heat and this can be converted to electricity with an above-average efficiency of 50%, then one MWh electricity costs €64. However, if the waste heat is fully utilized then the average price for the usable energy drops back to €32/MWh.

Challenges like high costs for construction, maintenance, and cabling, as well as finding additional space for new turbines, also make the case for wind turbines tricky. And it’s hard to see ways to significantly cut costs further as steel, concrete, and glass fiber are major components that do not seem to yield opportunities for significant price cuts. This thinking could be part of the reasons why the world’s largest wind turbine manufacturer, Vestas, now is taking a sneak-peek at PV as well.

PV may be on a better track than wind turbines. Though efficiency output seems to be stuck below 20% for fixed panels, PV benefits from its production resembling production of computer chips, hence allowing for much steeper cost reductions. Also, they are visually less intrusive from a distance than turbines as well as easier to install and maintain.

Efficiencies for concentrated solar thermal are much higher than for PV. E.g., compared to PV, Heliac’s solar thermal solution, which is the solution I’m most familiar with, converts more than four times as much incoming sunlight to energy. All of which can be utilized. In other words, if electricity and heat cost the same, concentrated solar can carry costs four times that of PV installations and still be competitive.

Comparing solar thermal to the more expensive wind turbines makes the case even better. If producing the same amount of electricity as the two postponed offshore wind turbine parks mentioned at the beginning of this article, Heliac’s solar collectors could reduce the average cost of energy from Vattenfall’s €64/MWh to below €20/MWh when producing electricity from solar-generated heat and selling the waste heat. The waste heat produced would amount to more than the total heat production from Denmark’s three largest coal-fired power plants today producing heat for district heating.

Interested in reading more? Please see the links to my other articles below. Additionally, a ‘Like’ from you will also be much appreciated as this should help direct more attention at the many business and climate opportunities the market for heat production offers.

Thank you for reading,

Jakob Jensen

HEAT is a series of non-technical, easy-read 3-minutes articles looking at heat’s role in energy production, its environmental impact, technologies for sustainable large-scale heat production, and some of the business opportunities these solutions generate.
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