for Seasonal Bridging, based on Glycerol-Water Mixture

With the increasing popularity of heat pumps in the building sector, I see the apparent discarding of waste heat or “wast cold” as a sad and annoying fact. While heat pump technology is generally more environmentally friendly than combustion engines, it still pains me that this apparent waste is so readily accepted. Many heat pumps even lack a reversing valve, limiting them to either heating or cooling only.

I understand why this is done. In my opinion, there is a lack of awareness that it would be possible to store thermal energy for months. Similar to the missing reversing valve, conventional heating technology does not want to be disturbed too much, possibly to avoid raising the bar for customers.

The topic of cooling will certainly become more important in the coming years, with hotter summers and the prevalence of home offices. Therefore, thinking about possible optimizations could be beneficial.

The goal of the idea is to store the thermal by-product of a heat pump so that it can be used several months later.

The energy from the storage can be used directly in the building or indirectly through the secondary side of the heat pump, which would mean higher flexibility in the temperature end-product at the cost of lower overall efficiency. Indirect use might better utilize the storage.

The fundamental limitations are the size of the storage, the optimal and limit temperatures of the heat pump, and losses, especially long-term losses due to the insulation of the storage.

A latent heat storage provides an almost constant temperature. Most of the storage capacity is in the latent range, where the storage material (Phase Change Material, PCM) absorbs energy to change its state.

The primary functionality of the storage only requires the following setup:
The secondary side of the heat pump and the latent heat storage are connected to each other via a heat exchanger with pipes. A medium is circulated in the pipes for heat exchange using a pump.

Thus, thermal energy can be transported from the heat pump to the storage in summer and from the storage to the heat pump in winter.
The heat that is excessive in the building in summer is pumped into the storage and then pumped back into the building in winter. The heat pump can regulate and provide desired temperatures, which would not be possible without it. The difference between outside or ground temperature and the comfort temperature is quite different in summer and winter.

Some latent heat storages use plastic balls filled with PCM. Here, the surface of the balls acts as a heat exchanger. Otherwise, plates or pipes are used.

In my research, I have already come across storages designed for this application, like the storages from Jekusol.
These storages already include insulation, heat exchangers, and are made from materials compatible with PCM.

Some PCMs are suitable for this application. Water first came to mind. Water has one of the highest heat capacities, especially in the latent range. However, the expansion of water and its strength when freezing make storage problematic. Additionally, I value long-term stability, and with water, there are concerns about bacteria and other foreign bodies proliferating in it. Furthermore, the freezing temperature of water cannot be practically shifted upwards, which would limit the latent range of the heat storage to 0 °C and below.

Common materials are various forms of paraffin. They have very high capacities and are long-lasting. However, their melting temperatures are typically between 40 °C and 60 °C, making them ideal for short-term heat storage. For the secondary side of a heat pump, this is too hot.

A substance that is abundantly available as a by-product of biodiesel production and has many positive properties for this application is Propan-1,2,3-triol (known as glycerin and glycerol). The heat capacity per volume is about 20% lower than that of water. Glycerol mixes excellently with water, and by adding small amounts of water, the melting point can be adjusted between about 20 °C and well below -20 °C.
This would allow adjusting the operating temperature of the storage throughout the year. In particular, the storage can be set near the ambient or ground temperature, which would minimize losses through the insulation.
Glycerol is hygroscopic, which counteracts the growth of foreign bodies. A negative aspect is its low tendency to crystallize. Depending on the storage geometry, this can cause problems in storing cold. However, this can be counteracted by introducing crystallization nuclei.

These properties can be used for secondary functionality. Small leaks between storage and heat exchanger are not catastrophic, but merely reduce efficiency. Leaks from storage to the outside world can be caught by a tray and thereby dehumidify the air in the room, protecting the room against corrosion and growth. A system that regulates the melting point by releasing and refilling the storage with water and glycerol could easily integrate these additional functionalities.

Assuming we have an oversized storage for an application, whose melting temperature is regulated to balance between losses through the insulation and the efficiency of the heat pump.
In my opinion, this would be the optimal solution to provide the thermal energy of a building. Thus, the COP of the heat pump could at least be doubled.
The demand for thermal energy will not be constant throughout the year and over the years, and will not exactly cancel out. If the insulation is too good, this could lead to oversaturation of the storage, which would in turn reduce the efficiency of the heat pump. For this exceptional case, an additional possibility to exchange heat with the outside air would probably be necessary.

Overall, I find this a great idea. An important point that I have not mentioned so far is the extent of the thermal energy needed in a building. Although latent heat storage can store much more thermal energy than a normal heat storage could, its energy density is still low compared to fossil fuels.
Thus, we are talking about volumes of PCM material in thousands of litres that would need to be accommodated.