New solution for combining bio- and solar energy for heating of low energy houses

*Background* - Bioenergy generated in stoves and fireplaces is the second largest source for heating after electricity in Norwegian homes and is popular all over Scandinavia. Traditionally wood stoves are installed for primary heating and because of indoor comfort. Houses are today built with low energy or passive house standard, which creates a mismatch between the instant heat power delivered by conventional stoves and the heat demand in the area that is directly exposed to the heat source. This challenge is addressed in the present project. *The main objective* of the project BioSol was to develop a new, save and competitive concept for heat transfer from a stove to a water-based heating system for domestic hot water preparation and space heating including the utilization of solar energy. A significant part of the heat demand can thereby be covered by renewable energy in terms of wood logs and solar energy with optimal system design. In contrast to existing, pressurized systems in the market the innovation should have a built-in safety, where the water jacket system is maintained at atmospheric pressure and water is only circulated when the thermal conditions are acceptable and secure. Drain-out should also occur during electric power failure. The aim was that the water jacket should be an easy add-on feature for wood stoves or fireplace inserts, not interfere with clean combustion or demanding new approval by the certifying body. BioSol was an Innovation Project for the Industrial Sector with five main partners: Jøtul, Aventa, Stansefabrikken Fredrikstad (SFF), Frost Produkt and Darlana University (Sweden), joining technical, R&D and manufacturing expertise with regard to stove/ fireplace inserts, solar energy systems, engineering and prototyping of steel plates, product design and research. During the first phase of BioSol, the development of the water jacket was concentrated on one popular wood stove model. This included heat transfer calculations, the design, the construction and laboratory testing of first prototypes. Much effort was used to enhance the heat transfer from the stove to the water jacket. This was followed up by comparative lab testing. Aventa, SFF and Jøtul had the main responsibility in this part of the project. Aventa has also elaborated a control algorithm for the operation and temperature regulation of the water jacket and implemented a beta version in the existing control system for their solar heating systems. In the product design work, Frost Produkt investigated several options how functionality, user friendliness, safety, esthetic design and optimal performance can be combined. For the optimal interaction with a solar heating system, we developed three system designs, how the BioSol concept can be integrated in small and medium-sized solar heating systems considering optimal production of renewable heat from solar- and bioenergy, heat storage and use: Two for conventional homes and one compact BioSol version for cabins with traditional standard. Full-scale pilot installations in detached houses were done and valuable learnings from user feedback was gained. Due to additional feed-back from the market, the water jacket has been modified so that it can be combined with several wood stoves or fireplace inserts from Jøtul. The reduced surface for radiative heat transfer is compensated with a new, added design for heat transfer from the bottom part of the flue pipe. Heat transfer studies were performed at different stages of the development at SFFs premises, Jøtul's R&D lab and the combustion lab at Dalarna University, at the latter also fluid gas emission were done. The aimed performance target of 3 kW heat transfer from the combustion to the water-based heating system could be reached in the second year. The "BioSol stove-top add-on concept" can principally be combined with most stoves/ fireplace inserts and increases the nominal efficiency of the stove. There are concrete plans to commercialise the innovation in collaboration with leading Norwegian companies. The first steps are related to the "BioSol flue pipe concept", which is independent on the stove model and can also be applied as retrofit unit for existing stove installations. Following dissemination events can be exhibited: IEA SHC Solar Academy Webinar and Workshop Journée R&D ADEME (2018), Solenergidagen 2017 and invited presentation at Oslo Innovation Week 2017, German-Norwegian Conference on Renewable Heating in Buildings arranged by Norsk-Tysk Handelskammer in Oslo (2017), Conference for "Solar Technology-Knowledge Exchange for Key Actors" (Kozani, Greece 2017) and at Norsk Solenergiforeningens Solvarmekurs og Solenergikveld (lokallag Bergen) (2016). BioSol included also that Aventa was leading Subtask A in IEA SHC Task 54 Price reduction of solar thermal systems from Oct 2015-Sept 2018.

Next to electricity is bioenergy in terms of log wood (firewood) the most used energy source for heating in Norwegian houses. The development toward low energy houses and passive houses creates a mismatch between the instant heat power delivered by a stove or fireplace, and the heat demand in the area of the house exposed to the heat from the source. This project presents a new way to transfer heat from a stove or a fireplace to an accumulator which combines high efficiency and negligible risks. The novel design is a hydraulic system without static pressure between the accumulator and the heat exchanger coupled to the stove. Water circulates in the heat exchanger only when heat should be transferred. If not the heat exchanger will be dry. The design removes all risks for accidents and damages due to boiling. The heat transfer rate will be controlled by dynamic flow rate control. The combination of log wood- and solar energy offers renewable energy for space and domestic hot water heating during all seasons. R&D challenges are related to the dynamics of the system. Clean combustion is required both when the heat exchanger is filled with water and when it is dry. The design of the heat exchanger should provide optimal transfer conditions during the whole combustion sequence. An advanced controller will be developed optimizing the function during different temperature conditions in the stove and the accumulator. The project will result in new products for all the partners. The heat exchanger is a new element to the Jøtul stoves. The product is an extension of the Aventa solar heating system for water- and space heating. A new process controller will also be made by Aventa. Heat exchanger and accumulators will be new products from Stansefabrikken, and Frost products will put important design elements in the product. All parties have essential roles, and will also increase the exposure of this product in the market through their various sales and distribution channels.