Optimized exploitation of PV electricity in a single-family house

How can electricity from a PV-system (photo-voltaic) best be utilized in a single family house? In a full-scale test setup with a real PV plant this project should examine how self-consumption of solar electricity in a typical family house, could be increased by use of batteries and water heating with a heat pump. How is such an energy system designed and controlled optimally in a single family house, and is there an economic benefit of the energy storage systems flowing to the PV marked in these years? The project should clarify those questions.

The storage devices (two batteries – and one heat pump) have been tested in a system that is built in Energy Flex Laboratory House at Danish Technological Institute. The instal-lation is supplied with electricity from rooftop solar cells, and the consumption is simulated minute by minute equivalent to the consumption of a single-family house.

The experiment with the heat pump showed that it was not so easy to store energy in the form of hot water as expected, due to a strongly decreasing efficiency (COP) with increasing hot water temperature. At elevated temperatures above 55°C, the system switched to direct electric heating with resulting low efficiency. The amount of excess electricity that could be absorbed from such system was found to be up to 5 kWh with the system under test (180 litres).

The experiment with the first (AC-coupled) battery system showed an excellent function of the battery itself, but a more dubious function of the inverter/charger unit. The main finding was that it is extremely important that there are no excessive losses in power electronics during the long periods when the system is running low load or is in standby mode. Overall storage efficiency was found to approximately 75 %.

The experiment with the other (DC-coupled) battery system showed that it was difficult to identify products with full documentation and functionality in relation to lithium batteries. It also proved that balancing self-consumption is not just a matter of power but also reactive and harmonic current that can cause serious error measurements. The overall storage efficiency was found to approximately 70 %, while the battery itself only lost a few %.

Besides the experimental work, an excel tool was developed as part of the project. The tool can help homeowners and retailers to calculate the technical and economic benefit of PV systems with battery storage. The results are that with the costs of today’s battery systems, it is barely profitable for Danish consumers to invest in their own PV battery, but with the reported cost trend for Li-Ion batteries, they will likely be so within a few years.

The results from the project is already being exploited by the partners and disseminated through articles, courses, web site and other channels. Lithium Balance has already - and will continue to use experience from the project, among other things concerning specifica-tions for suitable inverter types, and concerning safety in battery systems. Hopefully, the results will also encourage the development of better power electronics with much better part load efficiency than found in the tested products.

In the future, it may be possible to use the PV batteries to store cheap electricity - e.g. in the night. During normal operation, a solar PV battery with a shelf life of 12 - 15 years does not fully exploit the number of cycles it was designed for (more than 6,000 cycles for Li-Ion). Storage of grid electricity may therefore increase profitability in such investments.

Use of excess electricity for heating with a heat pump will also be very relevant in the future, when more and more households will switch to electricity based heating. Selection of the right components and control systems are some important issues for future work within this field.