Putin vs. heat pumps

The new tools of war are drones and energy. Russia is using both, the first against Ukraine, the second against Europe. The task for the Ukrainians is to expel this aggressor, and the challenge for the Europeans is to become energy independent. This is urgent, not only because Europe's oil and gas purchases pay for operating the Russian war machine, but also because their energy storage is running low, and the winter is here. Therefore, new energy supplies are quickly needed.

In this column, I will show how heat pumps can help to meet this need and how our process control profession can automate and optimize such a system.

Heat pumps use a working fluid to move heat from a low to a higher temperature level. This working fluid, having a low boiling point, evaporates into a low-pressure gas as it picks up heat from the surroundings that are relatively cold, but warmer the boiling point of the fluid. The gas, after being compressed, carries that heat obtained in the evaporator into a condenser where it is released into the warm heated surroundings of the heat pump. The air conditioner is such a device as it removes heat from our cooled buildings and rejects it into the relatively warm air outside. If, in the winter, the flow direction of the working fluid is reversed, the air conditioner can move the heat contained in the cold outside air into our homes and heat them (Figure 1).

Why do I mention these devices in connection with the present energy war? I do that because heat pumps are readily available on the market, they cost much less than furnaces or resistance electric heaters and can be quickly installed, plus they emit no carbon at all if the compressor is operated by green electricity. Naturally, it can also transfer heat both from air or water. This is an advantage in cold regions where the outside air temperature can drop very low, causing a drop in heat pump efficiency, while the ground temperature below the surface is relatively constant.

In the U.S., as we move from north to south, the efficiencies of heat pumps used for heating in the winter increase. In the south, sizing a winter heat pump, the beat capacity needed per square foot is about 30 BTU (kW/h = 3,410 BTU) and in the north, about 60 BTU (in Connecticut it is about 45 BTU/ft²). So, one weapon the EU members can readily use to weaken Putin's blackmail is to change the working fluid in their A/C units and install a reversing valve around their compressor (or just buy a new two-directional A/C unit).

Figure 2: In cities like Budapest the groundwater is warm enough to allow baths to stay open in the winter.

Another application of heat pumps is district heating. In many European cities, the hot (90°C TO 95°C) water for the buildings is centrally generated and is distributed by long pipelines. Compared to using individual furnaces in the served buildings, the cost of centralized district heating is about half, regardless of the heat source. On top of that, when the heat source is free, the costs are further reduced. Since in many city districts the distribution network is already in place, if the ground water is hot, it can replace the fossil fuel heat sources with such direct heat sources. To meet the winter heat load of a heated district, large heat sources are required, that exist in many areas where the ground water temperature is over 90 °C°. (Figure 2).

Figure 3: If the ground water temperature is under 90°C, geothermal heat-pumps are used to elevate the water temperature before distribution to the districts.

If the ground water temperature is below 90°C, heat pumps are needed. In both direct and heat-pump based district heating systems, the heat source is free, and the total system can also be free of carbon emission, if the pumps and compressors are also operated by green (geothermal or solar) energy. Today, out of the thousands of district heating systems around the world, unfortunately the heat sources of the majority are still fossil fuel. The proportion of the ones using geothermal energy is under 10% (Figure 3). If the EU focused on converting their many district heating systems, they could much reduce their energy needs.

Figure 4: The controls of a fully optimized geothermal district heating system.

The total energy needed to operate such district heating system, is the sum of the energy needed to operate three circulation loops (ground, working fluid and district water loops). In older systems, the two pumps and the compressor are often constant speed and manually controlled. In a fully automated and optimized system (Figure 4) the throttling of all three loops is by speed variation, which eliminates valve pressure drops. In addition the district circulating pump (P2) speed is set so that all individual building district supply valves CV-1, CV-2, etc. in Figure 4) are all kept under 90% opening.

The heat demand of the district is the sum of all the supply valves in it. A high signal selector selects the control signal that throttles the most open valve and sends it to a valve position controller (VPC) which compares that % opening with it's set point of 90% and throttles the P2 pump speed, which(increases the total district supply pressure to keep the most open valve from opening beyond 90%. and operates, keeps the openings of all under that value. The The flow in the district heating loop (F2) indicates the heat load on the district and informs the flow ratio controller (FrC), which throttles the ground loop (P1) speed to continuously hold the heat pump load (F1) at the optimum ratio with the district load F2) Finally, the speed of the compressor is throttled by the district supply temperature controller (TC) having a set point, which is continuously optimized by the overall system algorithm to keep the total cost of operation at a minimum.