Heat Recovery in Chillers and Their Link to Decarbonization Goals


The electrification of heating presents a significant opportunity to achieve decarbonization goals by reducing or eliminating the use of fossil fuels in traditional building systems such as boilers. The current geopolitical scenario has caused commodity prices to quickly rise and forced businesses, cities and countries to rethink their future dependance on fossil fuels and accelerate the conversion to sustainable alternatives.

Electric-operating heat pumps at the Ringsted District Heating facility in Denmark.

Most commercial and industrial facilities have separate cooling and heating systems. Chillers and heat pumps are electric-operated machines designed to transfer heat from one point to another. Chillers are used for cooling applications to remove the heat from a process (process cooling) or occupied space (comfort cooling) and usually reject the unwanted heat to the outside via an air-cooled condenser, cooling tower or fluid cooler, while heat pumps are used in heating applications to transfer heat from outside into occupied space or process.

Reversible heat pump systems, which combine cooling and heating in the same equipment, are commonly available in small systems such as residential or light commercial and their application is also climate-dependent (usually warmer or mild climates). Large commercial and industrial systems as well as smaller systems installed in colder climates usually rely on boilers to provide heating.

The Challenges of Implementation

Heat recovery consists of utilizing the unwanted heat from one system and applying it to another system. Combining cooling and heating systems together in one system is a clever approach to electrify and decarbonize the site, so the unwanted heat from the cooling system can be transferred to the heat system or vice-versa. It sounds simple in theory, but in practice it is more complicated.

 It is important to first understand how the cooling and heating systems operate, including variation in load sizes, operating temperatures and operating hours during the year. In existing installations, the first step is to understand the design operating temperatures for both cooling and heating systems and evaluate if the temperature of the heat rejection of one system can meet the design operating temperature of the other system.

In general, boilers can generate very high hot water and steam temperatures that are difficult to achieve solely by utilizing the heat rejection of cooling systems. In newer systems, a detailed evaluation of the operating temperatures of each system is strongly recommended during the design phase, so the difference between the design cooling and heating operating temperatures could be reduced without compromising each system and, consequently, increasing the possibility of transfer heat from one system to another.

Another key challenge is to match both cooling and heating loads during the same period, so the combined cooling-heating system can operate seamlessly. In most facilities, during the summer months, the demand for cooling will be higher than the demand for heating, and the opposite will occur during the winter months. Other facilities may have a very high demand of only one type of load, cooling or heating. Data centers, for example, have a high demand for cooling during the whole year and very limited need for heating during the same period. Most of the unwanted heat from the cooling systems in data centers cannot be used in the site and it is usually rejected to the atmosphere.

In most existing data center installations, finding a heating consumer close to the site to utilize the unwanted heat from the cooling system may be difficult or unfeasible depending on the required district heating or cooling infrastructure to transfer heat to the consumer. The latest trends in some European countries show that some data center facilities are now being installed close to high-demand heating consumers (residential, commercial or industrial), so the unwanted heat from their cooling systems can be economically transferred and utilized by those potential consumers.

In most of the United States, there is limited district heating and cooling infrastructure already installed which further reduces the feasibility of heat recovery solutions. The exceptions are some universities and colleges, where the district heating and cooling infrastructure is already available and heat recovery solutions should be evaluated and considered.

The oil-free centrifugal Danfoss Turbocor® compressor.

The Potential of Hybrid Solutions

Even though it is very difficult to completely match all the design requirements and synchronize the loads for both cooling and heating systems, there are several benefits of a hybrid solution approach where the heat recovery is implemented to partially address the need of the cooling and/or heating systems. Hybrid solutions can significantly improve the overall combined system performance and reduce both greenhouse gas (GHG) emissions and operating costs at a short implementation time.

In a hybrid solution, the heat rejection of the electric-operated cooling system could preheat the entering water in the boiler, and consequently, increase the efficiency of the boiler and the overall combined system efficiency. A reduction in fossil fuel consumption at the boiler will be realized since the amount of energy required to achieve the desired water or steam temperature is reduced due to the higher entering water temperature. The reduction of fossil fuel consumption will directly reduce the operating costs and, consequently, reduce the site’s greenhouse gas (GHG) emissions.

A practical example of utilizing electric-operating heat pumps is the successful case of Ringsted District Heating in Denmark. The district heat plant was redesigned in a collaboration between Ringsted, Geoclima and Danfoss with a focus on high-efficiency electric heat pumps that utilized the innovative technology of highly efficient oil-free centrifugal Danfoss Turbocor® compressors. In this project, the unwanted heat that was originally rejected to the atmosphere was used to generate hot water and re-introduced into the district heating network. Three heat pumps with dedicated heat exchangers were designed to maximize the efficiency. The result of this innovative solution was a very high energy savings with an improvement in the overall heat plant coefficient of performance (COP) of up to +21% and a reduction of approximately 97% in the SO2 emissions.¹

Utilizing heat pump technologies in new building design and retrofits will go a long way toward achieving the site’s decarbonization goals. In addition to reducing emissions, using electricity as a power source creates more energy-efficient cooling and heating systems, often resulting in significant savings for the facilities and increased comfort for its occupants.  Further reduction in greenhouse gas (GHG) emissions can be achieved when the site utilizes renewable sources of electricity such as solar or wind.

A cut-away image of the oil-free centrifugal Danfoss Turbocor® compressor.


About the Author

Jose Alvares is currently the vice president of sales and marketing for Danfoss Turbocor. He has an MBA degree with a focus on international business and strategic marketing from Mercer University (USA), an M.S. in industrial business from Londrina State University (Brazil), and a B.S. in mechanical engineering from Itajuba Federal University (Brazil). He has over 25 years of global experience in the HVAC-R industry and has held multiple positions in global sales, product management and marketing, application engineering, design engineering and R&D while living and working in the United States, Brazil and China.

About Danfoss

Danfoss engineers advanced technologies that enable us to build a better, smarter, and more efficient tomorrow. In the world's growing cities, we ensure the supply of fresh food and optimal comfort in our homes and offices, while meeting the need for energy-efficient infrastructure, connected systems, and integrated renewable energy. Our solutions are used in areas such as refrigeration, air conditioning, heating, motor control, and mobile machinery. Our innovative engineering dates back to 1933 and today Danfoss holds market-leading positions, employing more than 27,000 employees and serving customers in more than 100 countries. We are still privately held by the founding family. Read more about us at www.danfoss.com.

To read similar Heat Recovery System Assessments articles visit https://coolingbestpractices.com/system-assessments/heat-recovery.

Visit our Webinar Archives to listen to expert presentations on Chiller Technology at https://coolingbestpractices.com/magazine/webinars.

¹ Webinar about super-efficient heat pumps in Ringsted – DBDH, extracted from https://dbdh.dk/event/webinar-about-super-efficient-heat-pumps-in-ringsted/ .

July 2022