Heat transition in buildings - Why is it so difficult?

Author: Prof. Dr. Jochen Fricke, Cluster Energietechnik (As of: August 2016) Energetic building renovation much too slow, heating with PV and wind power not productive enough. Cheap oil leads to a renaissance of oil heating.

Substantial amounts of energy are needed to heat homes in Germany. There also plays a "luxury problem" an important role: the person-related living space is namely from about 7 m² / head (mostly only partially heated) after the Second World War until today quite continuously to 44 m² / head (mostly fully heated)  risen. Due to the energetic refurbishment of part of the old buildings, and new construction, the area-related heat demand was reduced from about 250 kWh/(m²∙a) in the 1970s  today to about 160 kWh/(m²∙a). The two opposite developments led to a flat maximum in the person-related annual heating energy consumption around the turn of the millennium of about 7500 kWh/(head∙a), which today has only decreased to about 7100 kWh/(head∙a).

From this, the total heating demand in Germany can be estimated at about 560 TWh (1 terawatt hour = 1 billion kWh) for a population of 80 million persons. This demand is covered by about 20 million heating systems. More than 70% of them are old. The coverage shares are 150 TWh from the approximately 5 million oil burners, 300 TWh from the approximately 13 million gas burners, 50 TWh from district heating and 70 TWh from renewables. Thermal energy from oil causes emissions of about 550 g CO₂/kWh in the building stock, natural gas of about 450 g CO₂/kWh, and district heating of about 130 g CO₂/kWh. If we neglect the (quite noticeable) CO₂ emissions when using renewables, we thus obtain heating-related emissions of about 220 million t CO₂ per year. This corresponds to almost a 30% share of energy-related CO₂ emissions in Germany. (It should be noted that for new heating systems, emissions are significantly lower, around 300 g CO₂/kWh for oil and 240 g CO₂/kWh for gas.)

The goal of the Federal Republic is to achieve a substantial reduction in CO₂ emissions in the buildings consumption sector as well. While it is to be hoped that the stringent defaults for the heat insulation and the heat supply  from environmental sources with new building projects are largely kept, the energetic reorganization of the existence comes only hesitantly forwards.

Once there  are fundamental discussions, namely around the lastingness of insulation measures. Desired would be inexpensive, non-combustible, non-toxic,  recyclable, lightweight and easy-to-install  insulation systems with low thermal conductivity and low CO₂-footprint. Available materials include polystyrene, polyurethane, glass/rock wool, mineral foams, paper shavings,  organic natural products and the highly efficient vacuum insulation panels (VIPs).  The choice is probably often made rationally, but certainly also strongly influenced by rumors (e.g. "the facade must be able to breathe", although facades are of course airtight). The decision whether to insulate at all depends very much on the subsidy conditions. In this regard, grants and tax incentives are now much more attractive than loans, with quasi-zero interest conditions.  

High renovation demand

The fact is that of the approx. 20 million German old building residential units, at most a proportion of approx. 1% is renovated annually in terms of energy - one therefore speaks of a century problem. Above all because of a limited craftsman potential, in particular the craftsman new generation, probably also in the future no substantially higher reorganization rate will be able to be reached. The renewal of the nearly 20 million heating systems is also making slow progress, with about 500,000 heating system replacements per year.  

There the question forces itself whether and if necessary like the heating-conditioned emission of approximately 220  million t CO₂ in the year otherwise noticeably to reduce can? The repeatedly voiced proposal is called Power-To-Heat (P2H). 

The expansion of photovoltaics (PV) in Germany led to an installed capacity of about 40 GW at the end of 2015. The installed capacity of onshore and offshore wind turbines was about 45 GW.  The power feed-in in this respect has led to considerable fluctuations in power supply, and even to negative prices on the power exchanges. Since thermal storage is much cheaper to implement than electrochemical storage, the proposal to  primarily use renewable "surplus" electricity for heating purposes has led to the  P2H concept.  

A similar P2H concept already existed in the past, when "night storage heaters" used primarily low-cost nuclear power electricity at night to heat high-temperature heat storage tanks. During the day, the stored heat was then  distributed throughout the home via free convection or by a fan. The primarily nuclear-fueled nighttime electricity storage heating system is probably passé.

Is Power to Heat the Solution?

So could regeneratively (i.e., also low in CO₂) generated  electricity make a significant contribution to building heating in the future? Particularly simple and inexpensive is the installation of a heating rod in the hot water storage tank.  However, thermodynamically preferable is the use of compression heat pumps for heat generation. For example, with a figure of merit or coefficient of performance of 3.5, an amount of heat of 3.5∙X joules is provided from an amount of electricity of X joules. Thermally insulated high-temperature ceramic storage furnaces with service lives of a few days are also eligible for P2H.

The annual PV electricity contributions fed into the grid are currently 38 TWh_el, of which the heating period from October to March accounts for about 25%, or about 10 TWh_el. Since there are only about 600 000 heat pumps in Germany, installed mainly in low-energy houses with a heating demand of about 3000 kWh_el, only about 1.8 TWh_el of this can be used and converted into about 6 TWh of thermal energy. The rest of about 8 TWh_el can only be utilized in heating rods. Thus, a total of about 14 TWh could be thermally generated, which would cover about 2.5% of the required heating energy. However, if only the PV "surplus" electricity should or can be used for this purpose, photovoltaics makes virtually no contribution to CO2 reduction in home heating.

The annual electricity yield from wind turbines of 80 TWh_el is much higher in the north than in the south and higher in winter than in summer. In the heating season, about 50% or about 40 TWh_el is available. These can be converted into the corresponding amount of thermal energy by means of heating rods, which corresponds to about 7% of the German heating energy demand. A prerequisite, however, is that the wind power generated primarily in the north can be distributed throughout Germany. This presupposes the realization of correspondingly powerful  north-south high-voltage lines.

The sobering realization is that P2H does help to cap electricity peaks. However, P2H - with the currently installed PV and wind power capacity and the lack of extra-high voltage lines - does not lead to any noticeable reduction in CO2 emissions from home heating. The "heat turnaround" in this respect is therefore not taking place for the time being, especially since natural gas and heating oil are the dominant heating energy sources with more than 75% and will probably remain so due to the low oil prices.