Reconstruction of Daugavpils heating plant No. 3 with installation of compressor heat pumps

PAS „Daugavpils siltumtikli“

Daugpilis, Latvia

2024-2025

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Heat produced using electricity, and how to obtain 4.5 kW of heat from 1 kW of electricity (COP 4.5) and something else.

Today, there are no heating networks in Lithuania that do not use condensing economisers for flue gases. This widely used technology has an Achilles heel: it operates with high temperatures of water returning from the network during the hot summer months and, as a rule, discharges condensate (at high temperatures) into the sewer system.

One of the problems (discharge of high-temperature condensate into the sewer system) was solved by AB Šiaulių energija in 2021. UAB ‘Gandras energoefektas’ designed and installed a system of compressor heat pumps, which, by cooling excess condensate, heat the network water, and the condensate itself, with a temperature of less than 15°C, is purified by the customer and used to feed the network.

The system has now been in operation for four years and meets all the customer's requirements. Since the condensate drained from the economiser into the sewer system is fed into the system, However, the amount of heat transferred to the network is highly dependent on fuel moisture content – with dry fuel, heat output drops by up to 30% (which, of course, has a negative impact on equipment depreciation). Our system is designed so that its performance is automatically adjusted from 15% to 100% depending on the volume of condensate, ensuring a constant COP value (heat pumps automatically stop and start (without intervention by personnel) when the volume of condensate decreases/increases).

We see that the amount of heat supplied to the network ranges from 250 kWh to 500 kWh, and the COP depending on the temperature of the supplied medium (and the customer, in addition to condensate from the KED, also supplies water from the steam boiler blowdown cooling system), varies from 3.9 to 4.2.

We are very grateful to AB Šiaulių energija for their trust. Although during the tender our competitors assured the customer that it would be impossible to achieve the required COP of at least 4.0 with such a low budget (in 2020, we won with a price of 218,682.00 EUR be PVM), it would be impossible to achieve the required COP of at least 4.0 with such a low budget. However, the results speak for themselves – four years of operation have proven that the decisions made were correct.

Unfortunately, no one in Lithuania was interested in this experience, even though over four years this system has generated 9,018,980 kW of heat and continues to operate, while the price of electricity on the exchange is falling to 0 EUR.

At the same time, the heating networks of Daugavpils (Latvia) were very interested in increasing the efficiency of the boiler house, where four 7.5 MW wood chip boilers and two flue gas condensing economisers are installed. After visiting AB Šiaulių energija in 2022, Daugavpils heating networks began to consider installing heat pumps at this boiler house, but were only able to announce a tender in 2024 (as the warranty period for the boiler house was coming to an end ). At the same time, the parameters of the hot water boilers (the maximum water temperature at the boiler outlet varies from 100°C to 110°C), as well as the limited financial capabilities of the customer (budget not exceeding €350,000.00) did not allow the installation of an absorption heat pump.

What the customer wanted:

Additional heat output from a single economiser of at least 380 kWh;
The COP of the system (taking into account the electricity consumption of heat pumps and all auxiliary equipment of the heat pump system) must be at least 4.0;
The amount of heat transferred to the network must be constant (it must not depend on fuel humidity and boiler performance);
The system must increase the condensate output in the economiser to minimise the need to feed water from the water supply to the economiser;
The condensate temperature (excess drained from the economiser into the sewer) must not exceed 25°C;
The efficiency of the economiser must not decrease.

Results after the implementation of the heat pump system designed by UAB Gandras energoefektas for one economiser:

The amount of heat supplied to the network is at least 450 kWh;
The COP of the system is between 4.4 and 4.6;
Heat is generated continuously, regardless of boiler performance;
Raw water feed has been discontinued;
The temperature of the condensate discharged into the sewer is 25-26 °C;

The presence of a second system in the boiler room consisting of two 7.5 MW boilers and a condensing flue gas economiser operating without heat pumps allowed us to compare the operation of the system with and without heat pumps in real time. It can be safely stated that the heat output in the CHP has not changed — we observed It is safe to say that heat generation in the CHP plant has not changed — we saw (observing briefly) that at an air temperature of +24 °C and a return temperature of 46 °C, the economiser with one boiler in operation (~7 MW*h) generated 0.7 MW*h of heat (not counting 0.46 MW*h on heat pumps). However, according to the customer, previously, at an air temperature of +24 °C and a return temperature of 46-48 °C, he had to turn off the economiser, as it produced 0.3 MWh of heat, and the cost of the heat produced was equal to the cost of the electricity consumed.

As mentioned above, our observations were short-term, as the heat pump system had only recently been launched. More accurate data on the operation of the KED with heat pumps may be available after the summer season.

Conclusions:

The short period of operation of heat pumps in Daugavpils from 2 April 2025 to 21 April 2025 (date of writing) confirms that the technical solutions chosen are correct and can be safely implemented in condensing economiser systems for wood chip-fired boilers.
An important argument is the 168.462 MW of heat generated during the 17 days of commissioning and operation of the heat pump system with a COP of at least 4.5, and the understanding that previously this heat was essentially discharged into the sewer system.